Fix for #36387, User Preferences "Addons" panel bogs down the whole interface.

The addons panel draw function calls addon_utils.modules() which in turn retrieves a list of fake modules from the script paths every time. This can become costly when network paths are included for
addons. Solution is to put the scanning process into a dedicated "refresh" function and disable it in frequently called draw and filter functions, i.e. in these cases the cached addons_fake_modules list
will be used instead.

Note that this may lead to invalid addon lists if script paths are changed (which is not working 100% without restart anyway according to Campbell). For this there is now a "Refresh" operator button in
the addons preferences. If necessary and feasible such forced refreshes can be added later too.

release/scripts/modules/addon_utils.py

@@ -35,7 +35,6 @@ error_duplicates = False
 error_encoding = False
 addons_fake_modules = {}
 
-
 def paths():
     # RELEASE SCRIPTS: official scripts distributed in Blender releases
     addon_paths = _bpy.utils.script_paths("addons")
@@ -51,7 +50,7 @@ def paths():
     return addon_paths
 
 
-def modules(module_cache):
+def modules_refresh(module_cache=addons_fake_modules):
     global error_duplicates
     global error_encoding
     import os
@@ -184,6 +183,11 @@ def modules(module_cache):
         del module_cache[mod_stale]
     del modules_stale
 
+
+def modules(module_cache=addons_fake_modules, refresh=True):
+    if refresh:
+        modules_refresh(module_cache)
+
     mod_list = list(module_cache.values())
     mod_list.sort(key=lambda mod: (mod.bl_info["category"],
                                    mod.bl_info["name"],
@@ -370,6 +374,9 @@ def reset_all(reload_scripts=False):
     """
     import sys
 
+    # initializes addons_fake_modules
+    modules_refresh()
+
     # RELEASE SCRIPTS: official scripts distributed in Blender releases
     paths_list = paths()