createMETS2: prefix PREMIS namespacing

Leaving PREMIS elements unprefixed resulted in any other unprefixed elements being interpreted as being FITS - for example, un-namespaced documents from external sources.

Fixes #7055.

Unfortunately this isn't (yet) a *real* fix - just a workaround. The outer document still uses a None namespace (for METS), so non-namespaced elements from emedded documents are now interpreted as being in the METS namespace instead of the PREMIS namespace. (This is also why `div` elements are interpreted as being `mets:div` elements, for example.)
This one does, yes - doesn't validate otherwise. It is unusual!