Avoid module call for unuse() for LMod and set $MODULEPATH directly

Followup to easybuilders#3557
No check for priorities is required as LMod correctly handles that
Also adds a check for an empty path which is technically possible and
handled by LMod like any other path

Author: Flamefire

easybuild/tools/modules.py

@@ -1443,6 +1443,21 @@ def use(self, path, priority=None):
                                ('<unset>' if cur_mod_path is None else cur_mod_path, new_mod_path))
                 os.environ['MODULEPATH'] = new_mod_path
 
+    def unuse(self, path):
+        """Remove a module path"""
+        # We can simply remove the path from MODULEPATH to avoid the costly module call
+        cur_mod_path = os.environ.get('MODULEPATH')
+        if cur_mod_path is not None:
+            # Removing the last entry unsets the variable
+            if cur_mod_path == path:
+                self.log.debug('Changing MODULEPATH from %s to <unset>' % cur_mod_path)
+                del os.environ['MODULEPATH']
+            else:
+                new_mod_path = ':'.join(p for p in cur_mod_path.split(':') if p != path)
+                if new_mod_path != cur_mod_path:
+                    self.log.debug('Changing MODULEPATH from %s to %s' % (cur_mod_path, new_mod_path))
+                    os.environ['MODULEPATH'] = new_mod_path
+
     def prepend_module_path(self, path, set_mod_paths=True, priority=None):
         """
         Prepend given module path to list of module paths, or bump it to 1st place.

test/framework/modules.py

@@ -1157,11 +1157,23 @@ def test_module_use_unuse(self):
 
         self.assertFalse(test_dir1 in os.environ.get('MODULEPATH', ''))
         self.modtool.use(test_dir1)
-        self.assertTrue(os.environ.get('MODULEPATH', '').startswith('%s:' % test_dir1))
+        self.assertTrue(os.environ['MODULEPATH'].startswith('%s:' % test_dir1))
         self.modtool.use(test_dir2)
-        self.assertTrue(os.environ.get('MODULEPATH', '').startswith('%s:' % test_dir2))
+        self.assertTrue(os.environ['MODULEPATH'].startswith('%s:' % test_dir2))
         self.modtool.use(test_dir3)
-        self.assertTrue(os.environ.get('MODULEPATH', '').startswith('%s:' % test_dir3))
+        self.assertTrue(os.environ['MODULEPATH'].startswith('%s:' % test_dir3))
+
+        # Using an empty path still works (technically)
+        old_module_path = os.environ['MODULEPATH']
+        self.modtool.use('')
+        self.assertEqual(os.environ['MODULEPATH'], ':' + old_module_path)
+        self.modtool.unuse('')
+        self.assertEqual(os.environ['MODULEPATH'], old_module_path)
+        # Even works when the whole path is empty
+        os.environ['MODULEPATH'] = ''
+        self.modtool.unuse('')
+        self.assertFalse('MODULEPATH' in os.environ)
+        os.environ['MODULEPATH'] = old_module_path  # Restore
 
         # make sure the right test module is loaded
         self.modtool.load(['test'])