[ADD] core: new magical odoo.upgrades package

This package point to first package found in `upgrades_paths` or
fallback to alias of legacy `odoo.addons.base.maintenance.migrations`.

closes odoo/odoo#32650

Signed-off-by: Christophe Simonis <chs@odoo.com>

odoo/upgrades.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+import importlib.util
+import os
+import sys
+
+from odoo.tools import config
+from odoo.modules.module import get_resource_path
+
+for path in config.get("upgrades_paths", "").split(","):
+    if os.path.exists(os.path.join(path, "__init__.py")):
+        break
+else:
+    # failback to legacy "maintenance/migrations" package
+    path = get_resource_path("base", "maintenance", "migrations")
+
+if not path:
+    raise ImportError("No package found in `upgrades_paths`")
+
+spec = importlib.util.spec_from_file_location("odoo.upgrades", os.path.join(path, "__init__.py"))
+module = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(module)
+
+# shadow module and register under legacy name
+sys.modules["odoo.upgrades"] = sys.modules["odoo.addons.base.maintenance.migrations"] = module