For two reasons: reliable signing and bodhi updates we need MBS to be able to tag the top-level module-build into a configurable tag after cg_import.

The destination tag should be based on the stream of the underlying platform or bootstrap module (first found) with a default tag used if no configured streams are found.

i.e., if httpd is built against bootstrap-f27, then it should be tagged into f27-modular-updates-candidate.

i.e., if httpd is built against bootstrap-foo, and we don't have a tag configured for foo, then it should be tagged into a generic placeholder tag, say modular-updates-candidate.