* only eval config scripts and apply overrides once per app

* move new resource behaviour to rebar_resource_v2 and keep v1

* cleanup use of rebar_resource module and unused functions

It makes things worse than if it weren't there.
Fixes issue #1726

find_apps didn't read config files so no hooks were in the app_infos,
and now that hooks are being done rebar needs plugins to be able
to run clean plugin hooks in deps.

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.

First find the rebar.config file, and if any data is found, check for
src_dirs config. If nothing is there or no app is found, default to src/

This makes rebar_prv_install_deps able to support more src_dir configs

This lets people who use custom directories layout without leaving the
.app.src in src/ to still see their app build.

Includes improvments and function documentation for all modules (in
alphabetical order) up to rebar_core, and may have included more in
other modules as I saw fit to dig and understand more of the internals.

Instead fetching and resolving src deps (which could depend on pkg deps)
and then pkg deps this patch combines the two into a single set of
iterations by level. The only difference between src and pkg deps in this
new install_deps is how their deps list is found -- from the config or
lock file for src deps and from the neighbors of the vertex for pkg.

Listed applications are simply ignored.

Allows for configuration time sub-application building.

- Solve "Uncaught error" failure in case of syntax error in app.src file.
- Print helpful information on the location of the syntax error.

The deps are sorted and merged, but the merge function merges lists, not
elements. This yields deps that are duplicated and ran for multiple
times.

We first add proper sorts so the keymerge is guaranteed to be fine, and
then do a dedup run to get rid of duplicates if they happen to be.