Extensions: default to paths in {BLENDER_RESOURCE_PATH_USER}/extensions

Replace: {BLENDER_RESOURCE_PATH_USER}/scripts/extensions
With:    {BLENDER_RESOURCE_PATH_USER}/extensions

This makes more sense as not all extensions are scripts.

scripts/modules/bpy_types.py

@@ -1320,9 +1320,14 @@ class UserExtensionRepo(StructRNA):
         """Return ``directory`` or a default path derived from the users scripts path."""
         if self.use_custom_directory:
             return self.custom_directory
-        import os
         import bpy
-        return os.path.join(bpy.utils.user_resource('SCRIPTS', path="extensions"), self.module)
+        import os
+        # TODO: this should eventually be accessed via `bpy.utils.user_resource('EXTENSIONS')`
+        # which points to the same location (by default).
+        if (path := bpy.utils.resource_path('USER')):
+            return os.path.join(path, "extensions", self.module)
+        # Unlikely this is ever encountered.
+        return ""
 
 
 class HydraRenderEngine(RenderEngine):

source/blender/blenkernel/intern/preferences.cc

@@ -242,15 +242,14 @@ void BKE_preferences_extension_repo_dirpath_get(const bUserExtensionRepo *repo,
     return;
   }
 
-  char subdir[16 + sizeof(bUserExtensionRepo::module)];
-  BLI_string_join(subdir, sizeof(subdir), "extensions", SEP_STR, repo->module);
-  const std::optional<std::string> path = BKE_appdir_folder_id(BLENDER_USER_SCRIPTS, subdir);
-  if (path.has_value()) {
-    BLI_strncpy(dirpath, path.value().c_str(), dirpath_maxncpy);
-  }
-  else {
+  /* TODO: support `BLENDER_USER_EXTENSIONS`, until then add to user resource. */
+  std::optional<std::string> path = BKE_appdir_resource_path_id(BLENDER_RESOURCE_PATH_USER, false);
+  /* Highly unlikely to fail as the directory doesn't have to exist. */
+  if (!path) {
     dirpath[0] = '\0';
+    return;
   }
+  BLI_path_join(dirpath, dirpath_maxncpy, path.value().c_str(), "extensions", repo->module);
 }
 
 bUserExtensionRepo *BKE_preferences_extension_repo_find_index(const UserDef *userdef, int index)

source/blender/makesdna/DNA_userdef_types.h

@@ -625,7 +625,7 @@ typedef struct bUserExtensionRepo {
 
   /**
    * The "local" directory where extensions are stored.
-   * When unset, use `{BLENDER_USER_SCRIPTS}/extensions/{bUserExtensionRepo::module}`.
+   * When unset, use `{BLENDER_RESOURCE_PATH_USER}/extensions/{bUserExtensionRepo::module}`.
    */
   char custom_dirpath[1024]; /* FILE_MAX */
   char remote_path[1024];    /* FILE_MAX */