FindPython: fix python compiler validation

Ensure also the cache of properties is erased in case of multiple
searches.

Fixes: #20626, #20627

Tests/FindPython/IronPython2/CMakeLists.txt

@@ -0,0 +1,22 @@
+cmake_minimum_required(VERSION 3.1)
+
+project(TestIronPython2 C)
+
+find_package(Python2 REQUIRED COMPONENTS Interpreter Compiler)
+if (NOT Python2_FOUND)
+  message (FATAL_ERROR "Fail to found Python 2")
+endif()
+
+if (NOT Python2_Compiler_FOUND)
+  message (FATAL_ERROR "Fail to found Python 2 Compiler")
+endif()
+if (NOT Python2_COMPILER_ID STREQUAL "IronPython")
+  message (FATAL_ERROR "Erroneous compiler ID (${Python2_COMPILER_ID})")
+endif()
+
+if(NOT TARGET Python2::Interpreter)
+  message(SEND_ERROR "Python2::Interpreter not found")
+endif()
+if(NOT TARGET Python2::Compiler)
+  message(SEND_ERROR "Python2::Compiler not found")
+endif()