This commit changes mainly the `rebar_paths` module by adding a
new `runtime` target. When using that target, `rebar_paths:get_apps/2`
will, for each project app, find all their `applications` and
`included_applicactions` and filter those that could be found
in rebar3's state.

Closes #2108. Changes the path to be printed to a relative path
to the current directory.

Also removes the `exports_not_used` warning from the default ones,
as Tristan suggested.

- Added support to rebar_prv_xref for filtering out whole module. A
  module may now be added to the list of xref_ignores in rebar.config or
  via -ignore_xref() module attribute
  - Added unit test for whole module filtering support to xref CT suite

- Only set paths that need to be put as a priority
- Clean up paths before leaving API mode

The first point accounted for some performance cost, but the latter one
explains the 40% overhead in test runs: since rebar3 calls rebar3 a lot
with a bunch of fake apps, and that the new mechanism for path handling
by default does not _remove_ paths, it just _orders_ them, we would end
up in a situation where as the tests ran, more and more fake paths would
get added to the VM.

By the time the run was over, all path handling would take longer since
more paths needed filtering every time. By resetting paths at the end of
an API run, we prevent a given 'project' from polluting another one's
runtime and performance once the API successfully returns.

Move path management out of rebar_utils manual code path function
handling (which we leave there for backwards compat), and centralize
them to allow easier coordination of paths between plugins and deps.

On top of path handling, do a check of loaded modules to only purge and
reload those that actually need it done in order to prevent all kinds of
weird interaction and accidental purge kills. It also allows the
possible cohabitation of both at once, with a "in case of conflict pick
X" as a policy

Changing path handling in providers also highlighted a bunch of bugs in
some tests and appears to fix some in other providers, specifically
around plugins.

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.

This can be reproduced by running xref analysis against a rebar3
plugin project which doesn't list rebar3 as an explicit dependency
-- calls to certain bundled modules ('rebar_state', 'rebar_api',
'ec_cnv', ...) will result in a failed pattern match as these modules
appear to have had their abstract code stripped.

When a given behaviour module does not exist, rebar3 brutally crashes.
This patch makes it so instead, there is a warning output menitoning the
missing behaviour, and this one is omitted from the unused function
calls check. This means that unused calls will instead be shown for the
module implementing a non-existing behaviour, as if no behaviour were
declared in the first place.

If xref analysis is being run by a dependency during its compilation
phase, the xref provider will try to add that deps' parents to the check
job while the ebin/ directories for them do not exist. This causes a
crash.

This patch makes it so directories are only added if they are existing,
preventing failure on any compile hook for dependencies and allowing
successful compilation as a best effort.

Some tricky changes in there but should be okay

- 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.

Also expose it in rebar_api

Add xref support for multi-app projects. Instead of only adding the
top-level ebin directory to the xref server use
rebar_state:project_apps/1 to enumerate the applications for the
project and add the ebin directory for each one.

* Add a provider for the xref tool for running cross reference
  analysis on a project. Most of the code has been ported directly
  from the rebar2 rebar_xref module with some modification and cleanup
  to support testing.
* Port over the eunit test suite from rebar2, but convert it to
  common_test. The testing is the same, but now the
  erlang term output is examined to determine if the test run is successful
  instead of scanning the console output for the expected strings.