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.
Integration tests are useful in detecting breakages but much slower so
let's only use one per command and install formula outside the
integration test framework (which is also slow).
