Before this commit, XBlock static assets were extracted during the
collectstatic process by iterating through all installed XBlock classes
and creating a separate directory for each. However, some packages
install many XBlocks that actually share the same static assets. The
most notable example is problem_builder, though we also see this with
schoolyourself and google_drive.

This commit uses the parent module name to do package asset lookup,
collapsing those cases down and eliminating duplicates. For a default
install of edx-platform XBlocks, this reduces assets generated from 31M
to 14M.