AptSource assumes packages are non-native

AptSource currently assumes that all packages it finds in the sources.list come with an upstream tarball. This isn't necessarily true, there are also packages (with dashes in their version) that don't come with an upstream tarball.

For example, Spamaps hit this issue with ensemble where the ensemble daily PPA had something like 0.5~daily4+ubuntu1 and bzr-builddeb assumed that meant it had access to the 0.5 tarball.