purge loaded code when it conflicts with project apps in tests

install plugins from the global config to ~/.cache/plugins

update relx version. fixes #434

io_lib:format/2 will throw badarg when trying to print rebar_resource…

Ignore the build directory

generalize the "test" special dir into an {extra_src_dirs, [...]} option

remove use of `code:load_abs/1` in favor of `beam_lib:chunks/2`

avoid repeated loading and unloading of modules

like `src_dirs`, `extra_src_dirs` are directories to be copied to
the `_build` dir and compiled. unlike `src_dirs` they are not added
to the .app specification

be able to use the specified edoclet in deps

fix usage string in rebar_prv_unlock

Quote atoms to allow names to have dashes.

Make standalone vs complete project template clear

Hex pkg improvements

- The rebar package index files have been moved off the default path and
  will require a new `rebar3 update`
- Caching of downloaded packages automatically takes place in a path
  relative to the CDN used
- The cache path is not shared with hex as we now write and modify data
  in there arbitrarily
- Basic tests plus the working set for more of them is included

when compiling ct dirs base compiler input dir on project root rather than profile root

rather than profile root

fixes #415

Some templates like OTP apps are usable within releases, so the text
does remain a bit vague, but hopefully less than before.

Fix comma style in app.src templates

update `rebar.config.sample` with correct config keys for `ct`, `eunit` and `cover`

`ct`, `eunit` and `cover`

remove uses of 'case catch'

Further restricting skipped deps warnings

improve log messages on upgrades