[cmake] Create FindPythonInterpreter for host information

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)

cmake/modules/FindPython.cmake

@@ -20,7 +20,6 @@
 #
 # PYTHON_FOUND - system has PYTHON
 # PYTHON_VERSION - Python version number (Major.Minor)
-# PYTHON_EXECUTABLE - Python interpreter binary
 # PYTHON_INCLUDE_DIRS - the python include directory
 # PYTHON_LIBRARIES - The python libraries
 # PYTHON_LDFLAGS - Python provided link options
@@ -76,23 +75,12 @@ if(KODI_DEPENDSBUILD)
   endif()
 
   list(APPEND Python3_LIBRARIES ${LZMA_LIBRARY} ${FFI_LIBRARY} ${EXPAT_LIBRARY} ${INTL_LIBRARY} ${GMP_LIBRARY} ${PYTHON_DEP_LIBRARIES})
-else()
-  if(CORE_SYSTEM_NAME STREQUAL linux)
-    if(HOST_CAN_EXECUTE_TARGET)
-      find_package(Python3 ${VERSION} ${EXACT_VER} COMPONENTS Interpreter)
-    else()
-      find_package(Python3 COMPONENTS Interpreter)
-    endif()
-  endif()
 endif()
 
 if(Python3_FOUND)
   list(APPEND PYTHON_DEFINITIONS -DHAS_PYTHON=1)
   # These are all set for easy integration with the rest of our build system
   set(PYTHON_FOUND ${Python3_FOUND})
-  if(NOT PYTHON_EXECUTABLE)
-    set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE} CACHE FILEPATH "Python interpreter" FORCE)
-  endif()
   set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
   set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
   set(PYTHON_VERSION "${Python3_VERSION_MAJOR}.${Python3_VERSION_MINOR}" CACHE INTERNAL "" FORCE)

cmake/modules/buildtools/FindPythonInterpreter.cmake

@@ -0,0 +1,48 @@
+# FindPython
+# --------
+# Finds Python3 Interpreter
+#
+# This module will search for a Python3 Interpreter
+#
+# --------
+#
+# the following variables influence behaviour:
+#
+# PYTHON_INTERPRETER_PATH - use external python not found in system paths
+#                           usage: -DPYTHON_INTERPRETER_PATH=/path/to/python3
+#
+# --------
+#
+# This will define the following variable:
+#
+#  PYTHON_EXECUTABLE - The HOST python executable
+#
+
+# We limit search paths to rule out TARGET paths that will populate the default cmake/package paths
+# Note: we do not do a find_package call as it will populate targets based on HOST
+# information and pollute the TARGET python searches when required for actual target platform.
+find_program(PYTHON3_INTERPRETER_EXECUTABLE NAMES python3 python
+                                            HINTS ${PYTHON_INTERPRETER_PATH} ${NATIVEPREFIX}/bin
+                                            NO_CACHE
+                                            NO_PACKAGE_ROOT_PATH
+                                            NO_CMAKE_PATH
+                                            NO_CMAKE_ENVIRONMENT_PATH
+                                            NO_CMAKE_INSTALL_PREFIX)
+
+if(PYTHON3_INTERPRETER_EXECUTABLE)
+  execute_process(COMMAND "${PYTHON3_INTERPRETER_EXECUTABLE}" --version
+                  OUTPUT_VARIABLE PYTHON3_INTERPRETER_VERSION
+                  OUTPUT_STRIP_TRAILING_WHITESPACE)
+  string(REGEX REPLACE "^Python (.*)" "\\1" PYTHON3_INTERPRETER_VERSION "${PYTHON3_INTERPRETER_VERSION}")
+
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(PythonInterpreter
+                                    REQUIRED_VARS PYTHON3_INTERPRETER_EXECUTABLE PYTHON3_INTERPRETER_VERSION
+                                    VERSION_VAR PYTHON3_INTERPRETER_VERSION)
+
+  if(PythonInterpreter_FOUND)
+    # We explicitly use a CACHE variable instead of a TARGET as execute_command is not
+    # able to use a TARGET - https://gitlab.kitware.com/cmake/cmake/-/issues/18364
+    set(PYTHON_EXECUTABLE ${PYTHON3_INTERPRETER_EXECUTABLE} CACHE FILEPATH "Host Python interpreter" FORCE)
+  endif()
+endif()

cmake/scripts/linux/Install.cmake

@@ -194,6 +194,7 @@ install(FILES ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/scripts/${APP_NAME}Config.cm
         COMPONENT kodi-addon-dev)
 
 if(ENABLE_EVENTCLIENTS)
+  find_package(PythonInterpreter REQUIRED)
   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