plugins: search for .a files if necessary

Summary:
on windows, plugins are loaded via .a files,
but those paths were not being searched when loading plugins

Test Plan: ./validate

Reviewers: Phyx, bgamari

Reviewed By: Phyx

Subscribers: RyanGlScott, rwbarton, carter

GHC Trac Issues: #15700

Differential Revision: https://phabricator.haskell.org/D5253