Add support for file to source-repositories

Adds a file type to source-repositories to allow individual files
to make use of source-repositories caching.

REPO_DIRECTORY has been rename to REPO_DEST since it is now sometimes
a file.

This change also fixes the incorrect path to the cache-url in the
tar type.

Change-Id: Ib79e41969c982a02f0235318d9f254b39c3c6d93