Update deps if their checkout doesn't match lock entry

Lock forwarding is what happens when the file on disk are on a different
version from what is specified in the lock file. Files on disk should be
updated to respect what's in the lock file.

A negative test has been added so that lock files that are outdated vis.
the underlying git and packages but match files on disk do not get
forwarded. This job is left to the `upgrade' command.

Run hooks for single application only once.

don't add `{extra_src_dirs, ["test"]}` to `test` profile

adding it results in ct test suites being compiled before they're
properly copied so changes made to suites between runs are not
reflected if a run fails before test dirs are copied

Properly initialize command parsed args in rebar state

methods for retrieving `src_dirs` and `extra_src_dirs`

update readme building instructions to prefer source

purge loaded code when it conflicts with project apps in tests

note that now ALL `src_dirs` across included profiles are
compiled. previously only the last included profile's `src_dirs`
were used

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