We were selectively requiring the tap.rb file in a few places for
performance reasons. The main method we were referencing was the
`Tap.cmd_directories` method which uses `Pathname` and the `TAP_DIRECTORY`
constant internally. `Tap.cmd_directories` is mostly used in the `Commands`
module and that is loaded very early on in the program so it made sense
to move that command to that module. To facilitate that I moved the
`TAP_DIRECTORY` constant to the top-level and renamed it to
`HOMEBREW_TAP_DIRECTORY`. It now lies in the tap_constants.rb file.
A nice bonus of this refactor is that it speeds up loading external
commands since the tap.rb file is no longer required by default in
those cases.
- Only two audits were using this: `audit_keg_only_reason` and `audit_text`,
and they weren't using any of its text processing methods, so there's little
reason to keep it around.
- The "`keg_only_reason` shouldn't contain 'HOMEBREW_PREFIX'" audit can easily
be replaced with a RuboCop since that's "just" text parsing.
- The "tests should invoke binaries with `bin/<command>`" audit had to stay as
a FormulaAudit because it requires accessing attributes about the Formula
like its name, aliases, which RuboCop can't get to, but it was easy to move the
singular "read the text in the file" line from `FormulaTextAuditor`.
