Extensions: enable the package management add-on when enabled

Package management has been moved into addons_contrib so the addon
can be enabled when enabling extension repositories.

This means users can test extension repositories without having to
install a separate add-on.

Additional work is still needed with the server before this is ready
for general testing though.

Part of #117286.

scripts/modules/addon_utils.py

@@ -32,6 +32,8 @@ def _initialize_once():
     for addon in _preferences.addons:
         enable(addon.module)
 
+    _initialize_ensure_extensions_addon()
+
 
 def paths():
     return [
@@ -606,6 +608,13 @@ def module_bl_info(mod, *, info_basis=None):
 # -----------------------------------------------------------------------------
 # Extensions
 
+def _initialize_ensure_extensions_addon():
+    if _preferences.experimental.use_extension_repos:
+        module_name = "bl_pkg"
+        if module_name not in _preferences.addons:
+            enable(module_name, default_set=True, persistent=True)
+
+
 # Module-like class, store singletons.
 class _ext_global:
     __slots__ = ()
@@ -805,6 +814,10 @@ def _initialize_extension_repos_pre(*_):
 
 @_bpy.app.handlers.persistent
 def _initialize_extension_repos_post(*_, is_first=False):
+
+    # When enabling extensions for the first time, ensure the add-on is enabled.
+    _initialize_ensure_extensions_addon()
+
     do_addons = not is_first
 
     # Map `module_id` -> `dirpath`.

scripts/startup/bl_ui/space_userpref.py

@@ -2021,10 +2021,9 @@ class USERPREF_PT_extensions(ExtensionsPanel, Panel):
         layout = self.layout
 
         row = layout.row()
-        row.label(text="The add-on to use extensions is disabled! See:")
-        row.operator(
-            "wm.url_open", text="Extension Add-on Repository", icon='URL',
-        ).url = "https://projects.blender.org/ideasman42/bl_ext"
+        row.label(text="The add-on to use extensions is disabled!")
+        row = layout.row()
+        row.label(text="Enable \"Blender Extensions\" add-on in Testing to use extensions.")
 
 
 class USERPREF_PT_extensions_repos(ExtensionsPanel, Panel):