WM: correct logic for resetting add-ons & caching meta-data

All add-ons were being scanned at startup, while this didn't cause
errors it was noticeable with extensions where any errors in the
manifest were being reported at startup, even when running with
factory-startup (including blender's own tests).

Address two issues:

- The logic to "reset" add-ons, so as to match the preferences when
  reverting or resetting preferences always ran on startup.
  This occurred because a check for Python being initialized was
  incorrectly used to detect that this wasn't the first time preferences
  were being loaded (regression in [0]).

- Resetting add-ons scanned all add-ons (including disabled add-ons) to
  ensure their module cache is up to date. Since this the cache is
  lazily initialized, it's simpler to set it as uninitialized as
  resetting the add-ons doesn't require the cached meta-data.

[0]: 497bc4d199

scripts/modules/addon_utils.py

@@ -536,8 +536,9 @@ def reset_all(*, reload_scripts=False):
     """
     import sys
 
-    # initializes addons_fake_modules
-    modules_refresh()
+    # Ensures stale `addons_fake_modules` isn't used.
+    modules._is_first = True
+    addons_fake_modules.clear()
 
     for path, pkg_id in _paths_with_extension_repos():
         if not pkg_id: