RFE: support Fedora style per-SRPM git repo

[cheese]

Fedora project maintains a git repo for each SRPM. For example, xen [1]. In each repo,

1. The specfile and all the patches and extra sources are put in the root directory of the repo.
2. A source file can alternatively be specified in a file called 'sources' and stored externally. This is usually upstream tarball that can be fetched from a URL.

What I wish is to use Planex to rebuild official Fedora rpms without moving the stuff around to follow the current Planex layout. Just initialize a Planex working directory and clone some Fedora package git repos to some subdirectory, and then run 'make' to download external sources and rebuild all the packages.

[1] https://src.fedoraproject.org/rpms/xen/