Pre-existing bug: split_namespaced_id used first-dot-split. For modules
with dotted IDs (e.g. lib-core.maps-test), this misclassified
'<dotted-module-id>.<tilemap-name>' as cross-lib reference.
Replaced with split_local_tilemap() that matches by exact current-
module-id prefix. Module-id can now have arbitrary dots.
Surfaced by P.3.1 reference test-module 'lib-core.maps-test' which
has a dotted module-id.
