fixes for plugins in gems/plugins/

fixes CNVS-15130

allows access to a plugin's models (during truncation for specs),
migrations, assets, periodic jobs, prepended routes, api routes (during
documentation), flagging as reloadable, spec docs, etc. once the plugin
lives in gems/plugins/ instead of vendor/plugins/

Change-Id: Ia7a2fdf5ad6258c5de962e982b6bf62ebaf4d075
Reviewed-on: https://gerrit.instructure.com/42287
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>