Handle multiple sources and patch queues

Until now Planex has only built packages consisting of a single upstream tarball or repository.   These changes adapt Planex to build packages which consist of an upstream tarball and one or more auxiliary tarballs or repositories containing patches or extra files.

In Debian terms, the single upstream repository case is similar to the 'native' package model, whereas the tarball + auxiliary repository case is similar to the 'quilt' package model.