Cleanup tool finding functions

There was a duplicate definition of `gocode`, and `gofmt` didn't correctly search `$PATH`, and might as well use the helper function `gopathBinOrPathItem`.
Ah, I see why you left it that way now.

However, several linux distributions (at least Archlinux and Gentoo, maybe more) place the bundled tools (`go`, `godoc`, `gofmt`) in `/usr/bin`.

It would be nice to have a workaround for this short of making some symlinks.