Duncan Coutts authored May 29, 2018 and Alexis Williams committed Jul 14, 2018

In preparation for adding support for additional target types (local and
remote tarballs, local and remote source repos), refactor the existing
readSourcePackage action so it can be more easily extended to support
the extra target types.

In particular, having to download things means there are advantages to
handling all the packags in a batch. Indeed for source repos there will
be quite a bit of sharing, so they must be handled as a batch.