[build] copy Python detection code into StandaloneOverlays.cmake (#31234)

This is to support workflows that do not rely on reading the main
CMakeLists.txt.

Based upon #30992, #31020

Addresses rdar://62245784

Author: edymtt

cmake/modules/StandaloneOverlay.cmake

@@ -115,6 +115,14 @@ include(SwiftConfigureSDK)
 include(SwiftComponents)
 include(DarwinSDKs)
 
+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()
 
 # Without this line, installing components is broken. This needs refactoring.
 swift_configure_components()