Improve gmxPythonDiscovery.cmake

Improve gmxPythonDiscovery.cmake

* Do a better job of urging CMake 3.15+ to use the requested Python
  installation when a user specifies Python3_ROOT_DIR.
* Adopt another CMake 3.15 feature: Python3_FIND_VIRTUALENV is much more
  readable than the older alternatives.
* Allow FindPython inputs to be set before entering the module (such as
  with `-D` command line arguments) by checking before setting.
* Clarify which parts of this CMake module apply to the main FindPython
  based detection and which parts exist to provide hints to other
  embedded / downstream detection.

Refs #3720