This case is _really_ slow even although it's something we encourage
people to run often and build systems often do. The `brew --prefix`
case is really fast because it's just in Bash so: let's pull the
`brew --prefix <formula>` case into Bash too.
This doesn't handle any edge-cases like `--installed` and the formula
detection is pretty simple.
Also, to make this behaviour consistent, never output `Cellar` paths
from the (Ruby) `brew --prefix`; we never want people relying on the
Cellar paths anyway, only output them if the formula wasn't installed
(where, arguably, using a Cellar path is even worse) and the speedup is
worth this deviation in behaviour.
