Revert "Remove which -> type -P alias."

This reverts commit e8e8164f348a0e8655e1d50a7a404bdc62055f4e. I
misread the original commit as adding the "which" package, but it only
adds it to base.nix. So then the original motivation (making it work
in subshells) doesn't hold. Note that we already have some convenience
aliases that don't work in subshells either (such as "ll").