Bug 1489340 - Handle XPT files as blobs of data in packager. r=froydnj

We don't actually ship XPT files anymore, but it's still useful for the
packager code to handle old Firefox versions. But for that, we don't
really need the complexity of "linking" XPT files in a single unit per
directory. We can just as well keep the XPT files intact, as long as we
retain individual interfaces manifest entries for each.

And since those entries used to be all merged into one, we now instead
group them all together in manifests (which also happens to make it
easier on unit test changes).

Differential Revision: https://phabricator.services.mozilla.com/D5221