Improve usability of the source-repositories cache

* Rename the cache directory for sources

Make the cache directory name match that of the element that creates
it.  This makes it easier to find and more logically implies what
element created the cache directory.

* Add REPONAME to repository cache directory

For better usability add the reponame to the directory created
to cache it.

* Add migration code

Move pre-existing cache directories of the old name to the new name.

Change-Id: Ic62f19acb1746888ccbdf95eebf6e653f3e66357
Merge "Improve usability of the source-repositories cache"