bitbake: Enhance the fetchers' support for local mirrors
Modify the try_mirrors() function to return the localpath of the fetched file
and update the data dictionary to reflect this.
Secondly the metadata files, lock and md5, should always be stored relative to
the ${DL_DIR} as it is possible that the localpath is a read-only directory,
for example in the scenario where there is a read-only file:// mirror.
