compose: Put the expanded treefile content in /usr/share/rpm-ostree/treefile.json

Since the treefile format now supports includes, we would need to
either include the whole chain, or just the expanded portion we use in
the compose.  This patch does the latter.

This should allow a client to take the same treefile and generate a
similar tree (if they want to reproduce with the same RPMs, those can
be extracted from the RPM database inside the tree).