Cache conflicted_modules results

Author: brandongc

reframe/core/modules.py

@@ -8,6 +8,7 @@
 #
 
 import abc
+import functools
 import os
 import re
 from collections import OrderedDict
@@ -1030,6 +1031,7 @@ def available_modules(self, substr):
 
         return ret
 
+    @functools.lru_cache
     def conflicted_modules(self, module):
         if module.collection:
             # Conflicts have no meaning in module collection. The modules