don't import .peru from local overrides

Summary:
We need to create `.peru` in local overrides, because it keeps
track of lastimports. But we don't want to import that dir into cache.
Take advantage of the new `excludes` param on `Cache.import_tree`.

Test Plan: New integration test that fails before this diff.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D56