Now that 'command-not-found' helper loading has been significantly
simplified and the startup time improved in the official Homebrew tap,
the workaround for script detection (and caching) can be removed.
As is the convention in prezto, we cache the command-not-found handler to
avoid incurring the performance penalty of loading ruby interpreter on
every call. This restores the 'Homebrew way' of loading command-not-found
handler.
Further, the formally recommended command lookup mechanism in Homebrew
(viz., `brew command command-not-found-init`) is ruby based and is super
slow. To avoid performance penalty, we `find` it ourselves from
`TAP_DIRECTORY` defined internally in Homebrew.
This also reinstates support for custom taps or non-standard Homebrew location.
Indrajit Raychaudhuri