[cmake] Restore CMAKE_MODULE_PATH after temporary modification

As this ends up in e.g. opm-common_config.cmake, calling
find_package(opm-common) will modify the CMAKE_MODULE_PATH
in a downstream module if not restored. Since we only need
this modification locally it is nice to restore the variable.

cmake/Templates/opm-project-config.cmake.in

@@ -120,13 +120,16 @@ if(NOT @opm-project_NAME@_FOUND)
       find_package(opm-common CONFIG)
     endif()
     # This is required to include OpmPackage /opm-common-prereq.cmake
-    set(CMAKE_MODULE_PATH "${CMAKE_MODULE_PATH}" @PREREQ_LOCATION@)
+    list(PREPEND CMAKE_MODULE_PATH @PREREQ_LOCATION@)
 
     # extra code from variable OPM_PROJECT_EXTRA_CODE
     @OPM_PROJECT_EXTRA_CODE@
     # end extra code
 
     include(OpmPackage)
     include(@opm-project_NAME@-prereqs)
+
+    # remove the temporarily added path again to not pollute downstream modules
+    list(POP_FRONT CMAKE_MODULE_PATH)
   endif()
 endif()