Always use the prefix "xml" for the XML namespace

Error was "xml.parsers.expat.ExpatError: prefix must not be bound to one
of the reserved namespace names"
Include any digest in a selections ID element

Before, the Python code only parsed a <selection> ID as a digest if
there were no other digests. But that's inconsistent with the way it
parses <implementation> IDs.

Also, Selection.is_available now returns false when there are no
digests, like Implementation.is_available does, rather than throwing
an exception.