Redudant hs-boot file checks are not very good
For example see test
T20030_test3 has a redundant boot file N.hs-boot but there is no warning for it.
There a few tests with redundant hs-boot files which are not caught by warnings about redundant boot files:
We could make this a bit better to catch cases where the module doesn't depend on the boot file itself and therefore warn it's redundant.