print help for namespaces

fix path storing for plugins, fixes #527

Add error message for bad provider hook, fixes #521

pr2relnotes takes merged PRs from the git log and generates
formatted release notes including links to the PRs themselves

plugins provider

catch error if config can't be parsed into dict to print error message

ensure all `src_dirs` and `extra_src_dirs` are handled properly

print nice error message if do is undef for provider

Export env var REBAR_BUILD_DIR in hooks, as rebar_dir:base_dir/1

OTP apps show proper output

REBAR_DEPS_DIR          = rebar_dir:deps_dir/1
REBAR_BUILD_DIR         = rebar_dir:base_dir/1
REBAR_ROOT_DIR          = rebar_dir:root_dir/1
REBAR_CHECKOUTS_DIR     = rebar_dir:checkouts_dir/1
REBAR_PLUGINS_DIR       = rebar_dir:plugins_dir/1
REBAR_GLOBAL_CONFIG_DIR = rebar_dir:global_config_dir/1
REBAR_GLOBAL_CACHE_DIR  = rebar_dir:global_cache_dir/1
REBAR_TEMPLATE_DIR      = rebar_dir:template_dir/1
REBAR_APP_DIRS          = rebar_dir:lib_dirs/1
REBAR_SRC_DIRS          = rebar_dir:src_dirs/1

autoconf compatible variables
(see: http://www.gnu.org/software/autoconf/manual/autoconf.html#Erlang-Libraries):
ERLANG_ERTS_VER              = erlang:system_info(version)
ERLANG_ROOT_DIR              = code:root_dir/0
ERLANG_LIB_DIR_erl_interface = code:lib_dir(erl_interface)
ERLANG_LIB_VER_erl_interface = version part of path returned by code:lib_dir(erl_interface)
ERL                          = ERLANG_ROOT_DIR/bin/erl
ERLC                         = ERLANG_ROOT_DIR/bin/erl

Export env var REBAR_BUILD_DIR in hooks, as rebar_dir:base_dir/1

Whenever the old shell got killed and an app got loaded prior, the whole
thing would silently drop output as the old 'user' process was replaced
while application master processes would keep the old one's pid in
their internal state.

To work around this limitation, make sure the apps are booted only after
the shell is replaced so that only the new `user` pid is used.

when running `ct` tests

fixes #513

Use the same dialyzer rebar.config options as rebar

* All configuration is moved to `{dialyzer, PropList}`
* `dialyzer_warnings` renamed `warnings`
* `dialyzer_plt_warnings` renamed `get_warnings`
* `dialyzer_plt_apps` renamed `plt_extra_apps`
* `dialyzer_plt_dir` renamed `plt_location`
* `{plt_location, local}` uses profile base directory
* `dialyzer_plt` removed
* `dialyzer_plt_prefix` sets prefix of PLT name (default "rebar3")
* `dialyzer_base_plt_apps` renamed `base_plt_apps`
* `dialyzer_base_plt_dir` renamed `base_plt_location`
* `{base_plt_location, global}` uses rebar3's global cache directory
* `dialyzer_base_plt` removed
* `base_plt_prefix` sets prefix of PLT name (default  "rebar3")

PLT files are always named `<prefix>_<otp_version>_plt` to match rebar.
`plt_prefix` and `base_plt_prefix` are not available in rebar, where the
prefix is always set to the project's top level application.

Include same files in PLT as rebar2

add config option artifacts

* Only include direct dependencies + base plt
* Error on missing direct dependency
* Always use beams in ebin instead of reading .app file (copying rebar2)
* Remove fake unknown warnings (will clash with unknown warning in 18.0)
* Remove redundant search for .app file (code path includes deps)

improve escriptize provider

Handle custom logger shell breakage

When swapping handlers, if a custom shell has been installed, it's
possible it'll take over the tty and other options. This may break
common operations that work on an otherwise regular shell, so we ignore
failures and let things work with the custom shell only.

set default providers before installing plugins

run pre and post hooks for release and tar providers

keep all_deps list in state created for building deps

fix bareness issues

- Crashes in providers lib when no providers in a namespace are bare
- Making sure bareness matches semantics; i.e. a bare provider is
  visible, a non-bare provider is hidden.

Fix badarg exception when setting group_leader for a dead pid

When setting up a shell some time elapses between listing the pids and setting
the group_leader. If the process exited during that time then
erlang:group_leader/2 will crash with badarg.

allow `src_dirs` and `extra_src_dirs` at the root of the rebar config

improve the rebar3 shell

Helps with integration efforts, but unfortunately can't support the
'-sname' and '-name' options, only '--sname' and '--name'.

Config files from the command line would expect the wrong format:

    {app1, [...]}.
    {app2, [...]}.

Instead of the correct sys.config format:

    [{app1, [...]},
     {app2, [...]}]

Not supported yet: the recursive file references documented in
http://www.erlang.org/doc/man/config.html

fixing plugin install and paths

Precedence still goes to shell_apps.