support user-defined plugins

Summary:
Look for plugins in `~/.config/peru/plugins` and
`/usr/lib/peru/plugins`, in addition to the builtins that ship with peru
itself. Get rid of the old rule that says you can't override existing
plugins -- that was from back when we allowed individual projects to
provide their own plugins, which we no longer do.

Test Plan:
We had some old testing logic for custom plugin paths, even though that
feature had been mostly removed. Repurpose that test to load plugins
from a fake home directory.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D141