modules: make the module hook more robust

The module hook would previously fail if there were no enabled module types.

- Instead of looking for a `KeyError`, default to empty list when the
  config variable is not present.

- Convert lambdas to real functions for clarity.

lib/spack/spack/hooks/module_file_generation.py

@@ -3,19 +3,19 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+import spack.config
 import spack.modules
 import spack.modules.common
 import llnl.util.tty as tty
 
-try:
-    enabled = spack.config.get('modules:enable')
-except KeyError:
-    tty.debug('NO MODULE WRITTEN: list of enabled module files is empty')
-    enabled = []
-
 
 def _for_each_enabled(spec, method_name):
     """Calls a method for each enabled module"""
+    enabled = spack.config.get('modules:enable')
+    if not enabled:
+        tty.debug('NO MODULE WRITTEN: list of enabled module files is empty')
+        return
+
     for name in enabled:
         generator = spack.modules.module_types[name](spec)
         try:
@@ -26,5 +26,9 @@ def _for_each_enabled(spec, method_name):
             tty.warn(msg.format(method_name, str(e)))
 
 
-post_install = lambda spec: _for_each_enabled(spec, 'write')
-post_uninstall = lambda spec: _for_each_enabled(spec, 'remove')
+def post_install(spec):
+    _for_each_enabled(spec, 'write')
+
+
+def post_uninstall(spec):
+    _for_each_enabled(spec, 'remove')