Allow using include_top to filter results even when there is a constant prefix

Previously, when seeing a `include foo/$(VAR)/bar.mk`, the include
tops wouldn't apply to filter down the results any further.

Allowing include_tops is useful here to prevent cycles in the
load graph.

Fixes: 236025787
Test: go test
Change-Id: I3ea445f222ee2283a68d4aca5002df750204076f