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 Homebrew/homebrew#19743.
Closes Homebrew/homebrew#21087.

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