This replaces all deprecated function usage by alternative ones based on
a version switch enacted at compile time, preventing all warnings.

This will likely introduce some possible runtime errors in using a
Rebar3 compiled on OTP-20 or OTP-21 back in versions 19 and earlier, but
we can't really work around that.

A bunch of dependencies have been updated to support OTP-21 without
warnings as well.

This is done through 3 main change groups:

- replacing `~s` by `~ts` in format strings, so that strings that
  contain unicode are properly printed rather than crashing
- adding the `unicode` argument to all function of the `re` module to
  ensure transformations on strings containing unicode data are valid
  instead of crashing (see issue #1302)
- replacing `ec_cnv:to_binary/1` and `ec_cnv:to_list/1` with matching
  functions in `rebar_utils`.

The last point has been done, rather than modifying and updating erlware
commons, because binary and list conversions can be a contentious
subject. For example, if what is being handled is actually bytes from a
given binary stream, then forcing a byte-oriented interpretation of the
data can corrupt it. As such, it does not appear safe to modify erlware
commons' conversion functions since it may not be safe for all its
users.

Instead, rebar3 reimplements a subset of them (only converting
atoms and chardata, ignoring numbers) with the explicit purpose of
handling unicode string data.

Tests were left as unchanged as possible. This may impact the ability to
run rebar3's own suites in a unicode path, but respects a principle of
least change for such a large patch.

The option {spec,Specs} is allowed in ct_opts, but
rebar_prv_common_test:test_dirs did not take into account that Specs
could also be a string only, i.e. not a list of strings.

Parse given test specs and add all spec- and suite directories as
extra_src_dirs in order to ensure that all these directories are
copied to the _build area and the suites are compiled.

Specs located in the project- or app root are explicitly copied to the
_build area in order to avoid recursive copying of the complete
directory tree.

This is a bugfix. It makes sure that the given path to a testspec is
translated so common_test will pick the spec from the _build
directory, and not from the source tree.

This is necessary in order to automatically get the testspec included
as an artifact (i.e. copied to the _build dir) in the case when it is
stored in another directory than 'test'.

Bug: option 'spec' is not specifically handled when merging options
from the command line with options from rebar.config. Due to this, if
the config specifies a 'spec', then this will take precedence over any
'dir' and/or 'suite' on the command line.

This commit takes special care of all options that can be used to
select tests - meaning that if any of the options 'spec', 'dir',
'suite', 'group' or 'case' are specified on the command line, then all
'spec', 'dir', 'suite', 'group' and 'case' options in rebar.config
will be ignored.

`rebar3 ct --spec foo.spec,bar.spec,baz.spec` now works

also added support for the `join_specs` flag on the command line

don't bother checking the profile in the path to the data dir

add definition of 'COMMON_TEST' macro to eunit provider

The idea is that given we accept arbitrary config items for CT, we
should similarly be able to pass unsupported options and keep things
running.

However for unsupported options, a warning is very useful to have.

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'

* remove partial support for ct `test_spec` until it can be done properly
  and warn if `test_spec` is present in test opts
* use new compiler functionality to reduce complexity of provider
* reduce command line options available to those that can be
  supported properly

* modify compiler interface to work on either application objects or
  directories containing source files
* compile all sources in `src_dirs` to the application `ebin` dir and
  all sources in `extra_src_dirs` to a directory mirroring it's
  position in the app's `_build` directory. for example, `apps/foo/more`
  would compile to `_build/default/lib/foo/more`
  for `extra_src_dirs` in the root of a project with multiple
  applications (so orphan directories that don't "belong" to an
  application) compile to `_build/default/extras/more`
* copy directories specified in `extra_src_dirs` into the `_build`
  directory so tools like `ct` and `xref` that expect source to be
  in a particular location still work
* clean compiled artifacts from all `extra_src_dirs`
* alter `eunit`, `ct` and `cover` to work with the new directory
  structure
* billions of new tests

this test almost certainly only fails under very limited circumstances
limiting it's utility. it is known to fail with some versions of rebar3
when this test is run via `rebar3 ct --cover`

to prevent any data from being overwritten

fixes #395

alongside their source (ie, if `some_tests` is a directory that
contains test suites beams resulting from compiling them will be
placed in `some_tests` in the appropriate `_build` directory