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.

[PERF-303] Integer XBlocks/XModules into the static asset pipeline.

This PR, based on hackathon work from Christina/Andy, implements a way to discover all installed XBlocks and XModules and to enumerate their public assets, then pulling them in during the collectstatic phase and hashing them.  In turn, the methods for generating URLs to resources will then returned the hashed name for assets, allowing them to be served from nginx/CDNs, and cached heavily.