Some test cases had to be modified in a kludgy way so that abstract specs made
concrete would have versions on them. We shouldn't *need* to do this, as the
only reason we care is because the content hash has to be able to get an archive
for a version.
This modifies the content hash so that it can be called on abstract specs,
including only relevant content.
This does NOT add a partial content hash to the DAG hash, as we do not really
want that -- we don't need in-memory spec hashes to need to load package files.
It just makes `Package.content_hash()` less prickly and tests easier to
understand.
7ab46e26b5