Fix MusicBrainz::Server reference to get_collator

'aliased' is a leaky abstraction, which introduces a *method* for the
aliased import. As such, doing 'use aliased Foo' followed by 'Foo::bar'
does not mean what the user would expect. Specifically, I've now had
to fully qualify ::get_collator with the MusicBrainz::Server namespace
to work around this.