Refactor: schemas: Use element-rule-location

Replace rule externalRefs in constraints.rng with nested grammars,
replacing the start block with element-rule-location. (The default is
element-rule).

Also align a couple of other differences between constraints-3.9.rng and
constraints-next.rng, as part of copying the new constraints schema
over.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>