[build] refactor Python detection code

Move detection of Python code into dedicate CMake module, so it can be
reused for workflows that do not rely on reading the main
`CMakeLists.txt`.

Builds upon #30992, #31020

Addresses rdar://62245784

(cherry picked from commit 7bc15b8)

Author: edymtt

CMakeLists.txt

@@ -892,14 +892,7 @@ if(NOT CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   endif()
 endif()
 
-find_package(Python2 COMPONENTS Interpreter REQUIRED)
-find_package(Python3 COMPONENTS Interpreter)
-if(NOT Python3_Interpreter_FOUND)
-  message(WARNING "Python3 not found, using python2 as a fallback")
-  add_executable(Python3::Interpreter IMPORTED)
-  set_target_properties(Python3::Interpreter PROPERTIES
-    IMPORTED_LOCATION ${Python2_EXECUTABLE})
-endif()
+include(FindPython)
 
 #
 # Find optional dependencies.

cmake/modules/FindPython.cmake

@@ -0,0 +1,8 @@
+find_package(Python2 COMPONENTS Interpreter REQUIRED)
+find_package(Python3 COMPONENTS Interpreter)
+if(NOT Python3_Interpreter_FOUND)
+  message(WARNING "Python3 not found, using python2 as a fallback")
+  add_executable(Python3::Interpreter IMPORTED)
+  set_target_properties(Python3::Interpreter PROPERTIES
+  IMPORTED_LOCATION ${Python2_EXECUTABLE})
+endif()

cmake/modules/StandaloneOverlay.cmake

@@ -114,6 +114,7 @@ include(SwiftHandleGybSources)
 include(SwiftConfigureSDK)
 include(SwiftComponents)
 include(DarwinSDKs)
+include(FindPython)
 
 
 # Without this line, installing components is broken. This needs refactoring.