Find python path properly for installed underlays.

This is the only change I need to work with underlays that are completely defined by `CMAKE_PREFIX_PATH` in an initial cache rather than environment variables persistently set by sourcing underlay `setup.xxx` scripts every time you cmake/make.

See parallel email thread in ros-sig-buildsystem. I think it would be very convenient (if for us, then I'm sure others as well) to support this and it doesn't break support of the former manual sourcing style.