Fix run-mirror writing wrong project directory

When run-mirror downloads a package whose download files have a
different base name to the download directory, run-mirror ends up
writing the wrong directory name, which causes 404's on the mirror.

In this patch we use the actual upstream parent dir name exactly.

Change-Id: I90ca0e280dc59d26e5cfc70c8c02feeb734d1ae9
Closes-Bug: #1294522