The Files sub-context allows us to attach metadata to files based on
pattern matching rules.
Patterns are matched against files in a last-write-wins fashion.
The sub-context defines the BUG_COMPONENT variable, which is a 2-tuple
(actually a named tuple) defining the Bugzilla product and component for
files. There are no consumers yet. But an eventual use case will be to
suggest a bug component for a patch/commit. Another will be to
automatically suggest a bug component for a failing test.
