[enhancement] Improve performance of module backend when detecting module conflicts

[brandongc]

Running a typical subset of NERSC's test suite with ~500 tests, there are ~30 unique values passed to this routine and the result of call for a given input is static. module commands can take a surprisingly long amount of time to return.

This change reduces the time for

reframe -C ./nersc-config.py -c checks --dry-run --mode checkout

from ~735s to <600s.

[brandongc]

It looks like some versions of python do not like the syntax.

Also, technically this introduces a potential memory leak as the cache is global and keyed on all of the arguments to the function including self. It doesn't look like this class gets instantiated more than once in normal use, but could be an issue.

A less concise but straightforward approach to achieve the same effect:

 import abc
 import os
 import re
-import functools
 from collections import OrderedDict

 import reframe.core.fields as fields
@@ -947,6 +946,7 @@ class LModImpl(TMod4Impl):
                               'this Lmod installation')

         self._extra_module_paths = []
+        self._conflicted_modules_cache = {}

     def name(self):
         return 'lmod'
@@ -978,18 +978,19 @@ class LModImpl(TMod4Impl):

         return ret

-#    @functools.lru_cache(maxsize=None)
     def conflicted_modules(self, module):
-        fp = "/global/u1/c/cookbg/reframe-nersc-tests/conflict_module_args.txt"
-        with open(fp, "a") as f:
-            f.write(str(module) + "\n")
         if module.collection:
             # Conflicts have no meaning in module collection. The modules
             # system will take care of these when restoring a module
             # collection
             return []

-        output = self.execute_with_path('show', str(module), path=module.path)
+        module_name = str(module)
+        if module_name in self._conflicted_modules_cache.keys():
+            output = self._conflicted_modules_cache[module_name]
+        else:
+            output = self.execute_with_path('show', module_name, path=module.path)
+            self._conflicted_modules_cache[module_name] = output

         # Lmod accepts both Lua and and Tcl syntax
         # The following test allows incorrect syntax, e.g., `conflict