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.