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.

- Move HOMEBREW_TAP_DIRECTORY to startup/config.rb because this file
holds more of the directory constants
- Rename `Commands.cmd_directories` to `Commands.tap_cmd_directories`
to better express that the commands come from taps

This file has the directory constants while the other one has regexes.
Just better organization.