PackageParser: Add support for caching parse results on the file system.

The exact scheme for serializing parse results is left undefined, and
will be implemented in a follow-up change.

Also adds a unit test for this functionality.

Test: PackageParserTest
Bug: 30792387

Change-Id: I5be6a3f97b5688d0a1aa9e024e946d590ad92c13
2 files changed