support user-defined plugins

Summary:
Add search directories for plugins. On Linux/Mac we use:

1. `$XDG_CONFIG_HOME/peru/plugins`
2. `/usr/local/lib/peru/plugins`
3. `/usr/lib/peru/plugins`

And on Windows it's:

1. `%LOCALAPPDATA%\peru\plugins`
2. `%PROGRAMFILES%\peru\plugins`

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. Manually test on Windows.

Reviewers: sean

Reviewed By: sean

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