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

Dialyzer output file

fix failure on missing OTP_VERSION file #403

check registry for missing package that may exist for mix

delete all symlinks when copying from project files to `_build` tree to prevent any data from being overwritten

to prevent any data from being overwritten

fixes #395

add escriptize project template

add all plugins, not those just built, to path