Simplify PointLocatorBase

`PointLocatorBase` used to use CRTP. However, this pattern is
unnecessary as the only real subclass it calls is Build, which does not
need templating. The base class does not have to call the
`PrepareForExecution` method, so it can provide its own features to
derived classes more easily.

Also moved `PointLocatorBase` out of the `internal` namespace. Although
it provides little benefit other than a base class, it will make
documenting its methods easier.