Re-fix building i18n on pristine systems

A patch to 0.8.8 reverted the behavior here, causing distribution builds to
fail. This patch should provide the intended behavior for both cases.

Testing Done:
I built Fedora packages both with and without PYTHONPATH set in the
environment. Prior to this patch, the build would fail due to being unable to
locate djblets.util.filesystem. With this patch, the build completes
successfully.

Reviewed at https://reviews.reviewboard.org/r/6165/