Improve filesystem repo caching

[isc-dchui]

Fixes #536

Improves the filesystem repo caching by proactively rebuilding cache and improving cache building performance by about an order of magnitude. Specifically:

• Lazy load the actual manifest of the module by deferring until install.
• Use mtime of the module.xml as a proxy for whether a module has been modified and perform smarter cache building based on that.
• Drastically speed up filesystem scanning using Python.
• Installing from a filesystem repo will trigger a quick cache rebuild to pick up any additions, modifications, and deletions.
• Add -rebuild-cache flag to repo command to manually fully rebuild (with purge) cache.

[isc-jili]

@isc-dchui This looks really good! I made a pass and few small comments