Extract nearest_ancestor into a module of its own

`PathUtilities` is mixed in in a bunch of places, but only one of those
makes use of the `nearest_ancestor` method.

Stop spraying it around and instead put it in a module of its own that
the controller (alone) includes.
Move CommandT::VIM::PathUtilities out of VIM namespace

Other than a reference to `VIM::pwd`, there's nothing in this module
that makes it belong in the `VIM` namespace (we should reserve that for
extensions to Vim's Ruby runtime API).

So, this commit moves it up a level and updates the `require` statements
(plus adds one missing one, and removes one unnecessary one).