[rego] Check store modules before skipping parsing (#5520)

* [rego] Check store modules before skipping parsing

Fixes https://github.com/open-policy-agent/opa/issues/5511

This change will cause the operation to be timed if the store
modules have already all been compiled and there are no
rawModules. This might be undesirable.

Signed-off-by: Charlie Egan <charlie@styra.com>