DB schema: add directory→tarball provenance information

We cannot reproduce a bit-by-bit identical tarball (and any other archive format: dsc, jar, zip, etc.) from the information stored in the DB, nor we want, given how brittle that is. Instead, we are going to store $somewhere tarball in the SWH objstorage before unpacking them.

But we still want to be able to link a directory in the DB, to the tarball that led to it. To that end, we need to add a table mapping directory entries (the dirs obtained by unpacking) to content entries (the original tarballs).

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Migrated from T62 (view on Phabricator)