this fixes the issue where using 'rebar3 plugins upgrade P'
would result in a plugin's .app file having an empty
modules list. The code this replaces hadn't been touched
since rebar3 3.0.0-beta.3 and a lot of improvements have
gone into dep handling and the compiler since then. This
change should take advantage of those changes.

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.

Requires a rework of other test suites using the same dep-handling
mechanism.