Properly define tapped formulae in update

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.
Closes #21087.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>