The recent work to move Djblets over to the modern Python

importlib.resources and importlib.metadata modules regressed the

installation of extension static media. When checking if a resource

exists, we were using .is_file(), which only returns True if it's af

file and not a directory. This failed on the static directory.
This change switches this to a call to .exists(), fixing static media

installation.
Unit tests were added to ensure this doesn't break in the future.

Unit test pass.
Deployed this fix to reviews.reviewboard.org and fixed static media

installation for Power Pack.