Make requests python package to be detected in the same way as numpy

This solves missing requests package reported on the systems where it's
located in dist-packages rather than in site-packages.

To do this there's now a helper macros which handles both requests and
numpy now and could be used for more packages in the future.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D686

source/creator/CMakeLists.txt

@@ -539,7 +539,7 @@ if(UNIX AND NOT APPLE)
 			# Copy requests, we need to generalize site-packages
 			if(WITH_PYTHON_INSTALL_REQUESTS)
 				install(
-					DIRECTORY ${PYTHON_LIBPATH}/python${PYTHON_VERSION}/site-packages/requests
+					DIRECTORY ${PYTHON_REQUESTS_PATH}/requests
 					DESTINATION ${TARGETDIR_VER}/python/${_target_LIB}/python${PYTHON_VERSION}/site-packages
 					PATTERN ".svn" EXCLUDE
 					PATTERN "__pycache__" EXCLUDE           # * any cache *