FindPythonLibs: Do not try to find the interpreter (#13794)

The parent commit taught FindPythonLibs to try to find PythonInterp
unconditionally.  Some projects may want the libraries of a specific
version even when the corresponding interpreter is not available.  Drop
the internal use of FindPythonInterp and just use the versions from it
if it happens to have been found by the project first.  That will allow
projects to get a consistent version when they want both but not
otherwise force them to find the interpreter.
Merge topic 'different-python-header-libs-exe-0013794' into next

bf28ff1f FindPythonLibs: Do not try to find the interpreter (#13794)