- robocopying a directory into another directory recursively expects the
  directory name to be properly mapped onto the destination, otherwise
  all the files are copied into the given path. This patches things so
  a directory-to-directory robocopy works as expected in a linux mindset
  so tests pass
- the test for canonical paths didn't expect a windows environment at
  all; the test (and library) is modified to be consistent in that
  environment: always with a native format and with proper support of
  drive letters.

This reuses the trick used within OTP to pick within old and new shell.

The 'user' structure is the same for all cases (escript, escript + dumb
TERM, unstable install, unstable install + dumb TERM), so we take it
down first.

Then we boot the TTY driver, which fails if TERM=dumb, in which case we
boot the retro-style usr. If it worked, we shut down the driver again,
and boot a modern shell structure.

This avoids all warnings and seems to work in all cases.

This is quite the hack.

This requires to detect the current shell running; if it's the new
shell, business as usual.

However, if it's the old shell, we have to find a way to take over it
and drive IO. This requires a few steps because:

- the old shell does not let you be supervised intelligently (it uses
  supervisor_bridge, so killing the child is not a supported operation
  from the supervisor)
- the old shell ignores all trappable exit signals except those coming
  from the Port in charge of stdio ({fd, 0, 1})
- the old shell shuts down on all exit signals from the stdio Port
  except for badsig, and replicates the shutdown reason otherwise
- An escript does not tolerate the `user` process dying (old shell) for
  any non-normal reason without also taking the whole escript down
- Booting in an escript has an implicit 'noshell' argument interpreted
  by the old shell as a way to boot the stdio Port with only stdout
  taken care of

Because of all these points, we have to kill the old `user` process by
sending it a message pretending to be the Stdio port dying of reason
`normal`, which lets it die without triggering the ire of its
supervision tree and keeping the escript alive. This, in turn, kills the
old stdio port since its parent (user.erl) has died.

Then we have to boot our copy of user.erl (rebar_user.erl) which
conveniently ignores the possibility of running the stdio port on stdout
only -- always using stdin *and* stdout, giving us a bona fide old-style
shell.

A known issue introduced is that running r3:do(ct) seems to then kill
the shell, and r3:do(dialyzer) appears to have an odd failure, but
otherwise most other commands appear to work fine.

if these directories actually exist they'll be added to the path ahead
of the release/standard distribution directories and they'll break eunit
and/or ct execution

fixes #950

this allows repeated test suite names across apps without conflicts

previously rebar3 dropped suites declared at the root of the project (via
`--suite=whatever_SUITE' probably) and warned. this was because the compiler
would recursively copy and compile everything in the directory indicated by
the test suite. this changes the copy mechanism to only copy erl source files
and directories that end with `_SUITE_data' into the `extras' dir in `_build'

This lets a plugin define templates to be loaded:

    $ rebar3 new
    ...
    proper (plugin): A basic PropEr suite for an OTP application
    ...
    $ rebar3 new help proper
    proper:
            plugin template (...)
            Description: A basic PropEr suite for an OTP application
            Variables:
                    name="suite" (...)
                    ...
    → rebar3 new proper fakesuite
    ===> Writing test/prop_fakesuite.erl

In this case, proper is a fake template file I've put by hand in
_build/default/plugins/rebar3_proper/priv/<somename>/, meaning it will
only work as far as it's called from the project's root.

The priority order of plugins is now .config > plugin > built-in, such
that someone could ensure plugins do not crush their own private
templates, but also that custom or plugin templates do overtake built-in
ones. It used to be Built-in > .config only.

Templates are searched for recursively in the priv/ directory of
plugins.

This should fix #955

The test is implicit as a bad index previously silently crashed rebar3.
By adding the bad index to the `new` suite's files, we can show that
things keep running.

this restores compatibility with rebar2 and erlang.mk

Full rewrite, code should be understandable now.

- proper segregation of comparison between tuple terms and non-tuple
  terms. Guards weren't specific enough and that meant the wrong clauses
  of guards would be triggered
- proper deduplication of entries in the list. An additional N passes
  are required (we co-opt the reverse step to be more efficient) because
  while the original lists:umerge easily removes dupes, this is
  requiring more logic here since `[a,{a,b},{a,b,c},a,{a,b,c}]` is a
  possible interleaving and we'd want `[a,{a,b},{a,b,c}]` -- comparison
  of direct neighbours isn't enough.

The previous default meant that verbose output would not be emitted.

When compiling a dependency with a MIB file the generated hrl file is left in
the root project directory in a file called "include". This has the perverse
effect of messing up the search path for include files causing any dependencies
with files in their "include" directory to fail to build after that.

Relx config need new values at the end of the list and
profile additions prepended. So now in apply_profile in
the case it is a relx config we prepend the config list.
This does, however, mean that a profile release can not
'extend' a release defined in the default profile since
the one being extended would have its definition after
the profile release and thus relx would not be able to
use its values for extending.

The checking of flags and the parsing of arguments is separated up.

when `ct_opts`, `eunit_tests`, `eunit_first_files`, `ct_first_files`, `erl_first_files`,
`eunit_compile_opts`, `ct_compile_opts` and `erl_opts` have values that are single non-list
terms warn and try wrapping them in a list when processing them in the `eunit` and `ct`
providers

In some cases (nested includes?) paths end up in such a way that joining
them breaks up and hard-crashes rebar3. This patch specifically handles
this scenario to fix things by avoiding passing empty lists to
filename:join.

Application masters are booted at the root of apps, and take over the
group leader role to redirect IO. To cut the chain short and properly
have their role inherited, they are their own leader, and keep a
reference to the old leader in their internal state, which we cannot
change.

This is done so process ownership to a given application can be
established, and allows to properly clean up resources outside the
supervision tree when an app is shut down.

This patch goes around and finds all processes whose group leaders are
application masters older than the new `user' process booted by the
shell providers, and swaps them with that new `user'. This lets the
application masters survive, and fixes the blocking IO issue
(resolving issue #899)

This may mean an incomplete clean up is down on application shutdown,
but that seems like a fair compromise.

Adds the ability to run an escript before starting the apps and
interactive shell for a project. This is intended to improve the
local development experience for projects by providing an easy way to
run companion services (mock rest APIs, databases etc) that the
project relies on.

This patch also adds {shell, Defaults} to the rebar config file so
that a project can supply default values for many of the new or
improved 'rebar3 shell' options:
* {apps, OTPApps}
* {script_file, EscriptFileName}
* {config, ConfigFileName}

The order of option precedence is command line, rebar.config, relx.

This uses cth_readable to:
- silence error_logger output to the shell unless a test fails
- silence ct:pal output to the shell unless a test fails

I have currently not baked in any way to disable this behaviour, but I
figured if it is required, there is time to do it before the final
3.0.0 release.