Be more careful when injecting sibling resource dirs.

Two changes:

1. Prevent multiple bases that are siblings from injecting the
   same 'resources' sibling multiple times.

2. Only inject if the dir exists.

This mitigates a problem that was exposed by a bug in our
BUILD.bootstrap, whereby we hadn't registered scala_tests in
our tests/ source root. In this case the base is set to the
target's parent dir...