Prevent brew casks from finding formulae at tap root

[ThatsJustCheesy]

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Currently, brew casks will print names of formulae defined at the root of a tap repository. e.g. for this tap:

$ brew tap cartr/qt4
# …
Tapped 23 formulae (43 files, 222.1KB).

$ brew casks | grep 'cartr/qt4'
cartr/qt4/automoc4
cartr/qt4/coin@3.1.3
cartr/qt4/cuty_capt
cartr/qt4/ezlupdate
# …

I'm assuming this is a bug, so here is my best attempt at a fix.

All tests pass, but I'm not sure any exist for brew formulae and brew casks. From my manual testing, they appear to now work as I would expect.

[MikeMcQuaid]

Makes sense to me. Thanks again @ThatsJustCheesy!