[cmake] Create FindPythonInterpreter for host information

[fuzzard]

Description

Simplify existing FindPython and split host/target specifics

Motivation and context

This splits out host find information from the regular target find module. This allows us to not mix and check for host executable status, and only explicitly search for the host interpreter when required (eg ENABLE_EVENTCLIENTS=ON)
I will mention, this can NOT be a TARGET like everything else im heading towards for the fact that execute_process() can not use TARGETS. Its considered too low level, so a cache variable is the only way when combined with execute_process(). It could be changed to an add_custom_command, but i dont think its worthwhile for a single edge use case

@wsnipex ive been tinkering with the FindPython module, and moving the host stuff out would simplify my life greatly.
Ive got down the PLATFORM_REQUIRED_BUILDTOOLS route here (similar to req/optional dep platform variables), however i thought a little about this later, and it may just be simpler to make an explicit call

core_require_dep(PythonInterpreter)

at the following

xbmc/cmake/scripts/linux/Install.cmake

Lines 206 to 209 in 0f28947

	if(ENABLE_EVENTCLIENTS)
	execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "from distutils.sysconfig import get_python_lib; print(get_python_lib(prefix=''))"
	OUTPUT_VARIABLE PYTHON_LIB_PATH OUTPUT_STRIP_TRAILING_WHITESPACE)
	# Install kodi-eventclients-common BT python files

What are your thoughts?

How has this been tested?

locally macos with forcing certain paths to trigger the FindPythonInterpreter call

What is the effect on users?

N/A

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[fuzzard]

Have simplified this. We only need the find_package call to be made when ENABLE_EVENTLCLIENTS is done as part of the linux install.

[garbear]

Splitting host and target is a good idea. I took at look at the new CMake file and didn't see any problems.