make brew update define tapped formula properly

a tapped formula is a ruby file present:

- in the root of the tap
- in directory of the tap called Formula
- in a directory of the tap called HomebrewFormula

and nowhere else. this corrects an overzealous definition of tapped
formula in the updater. (the correct definition has been in Pathname
since e613cbe5783cea2abb8100b56c22126a1ab6b9f2)

refs #19743