Co-authored-by: Fred Hebert <mononcqc@ferd.ca>

- 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.

top level default profile plugins are installed in the run_aux
function. This commit removes the plugins from the plugin opts
of an application that is also at the top level and skips the
default profile when installing top level plugins in
project_apps_install so they aren't handled twice.

Additionally, before handling a plugin the state's list of
known plugin apps is checked and the plugin is skipped if it
has already been handled.

Based off a macro by @okeuday at https://github.com/erlang/otp/pull/1783

The path reloading of plugins had been fixed properly, but the problem
is that the paths it was using to re-load only considered the current
compile step, rather than the overall state of plugins. As such, the
reloaded paths after plugin compilation only reloaded the *latest*
plugin and not the other ones.

This fix forces the addition of all built plugin paths to the code paths
after a plugin compile job is run. This ensures that the path is clean
for initial plugin deps (only add those that are required), and is
re-made total after the fact (add all the plugins possible).

This commit also includes a system tests suite that can be run
optionally; the problem with this plugin mechanism was impossible to
find through mocked dependencies, and a working counterexample was
provided to us. The systest suite can be run against real projects
without conflict to make sure no regressions are hit.