Previously, superenv did not try to filter -I or -L flags
based on the list of requested dependencies; as a result, buildsystems
which opportunistically discover Homebrew-installed libraries were able
to link against them even under superenv.
This adds a list of all requested dependencies to the superenv environment,
and compares all -I and -L flags against those; any Cellar and opt paths
found which resolve to unrequested dependencies are filtered out.
4fd5c5c159