[FIX] core: correct imports from `odoo.addons.base.maintenance.migrations`

This legacy package was supposed to be an alias to `odoo.upgrade`.
However, depending on how your import its sub-packages (and in which
order), we were ending with the module being loaded multiple times,
breaking the expectation of a singleton.

```python
In [1]: from odoo.addons.base.maintenance.migrations import util as m1

In [2]: import odoo.addons.base.maintenance.migrations.util as m2

In [3]: m1
Out[3]: <module 'odoo.upgrade.util' from '/Users/chs/devel/odoo/odoo/stable/odoo/addons/base/maintenance/migrations/util.py'>

In [4]: m2
Out[4]: <module 'odoo.addons.base.maintenance.migrations.util' from '/Users/chs/devel/odoo/odoo/stable/odoo/addons/base/maintenance/migrations/util.py'>

In [5]: from odoo.addons.base.maintenance.migrations import util as m3

In [6]: m3
Out[6]: <module 'odoo.addons.base.maintenance.migrations.util' from '/Users/chs/devel/odoo/odoo/stable/odoo/addons/base/maintenance/migrations/util.py'>

In [7]: m2 == m3
Out[7]: True

In [8]: m1 == m3
Out[8]: False

In [9]:
```

Now, with this import hook, we ensure that the modules imported from
`odoo.addons.base.maintenance.migrations` are aliases to ones imported
from `odoo.upgrade`.

```python
In [1]: import odoo.addons.base.maintenance.migrations.util as m2

In [2]: m2.__name__
Out[2]: 'odoo.upgrade.util'
```

closes odoo/odoo#69386

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

odoo/modules/module.py

@@ -101,6 +101,33 @@ class OdooHook(object):
 
         return sys.modules[name]
 
+
+class UpgradeHook(object):
+    """Makes the legacy `migrations` package being `odoo.upgrade`"""
+
+    def find_module(self, name, path=None):
+        if re.match(r"^odoo.addons.base.maintenance.migrations\b", name):
+            # We can't trigger a DeprecationWarning in this case.
+            # In order to be cross-versions, the multi-versions upgrade scripts (0.0.0 scripts),
+            # the tests, and the common files (utility functions) still needs to import from the
+            # legacy name.
+            return self
+
+    def load_module(self, name):
+        assert name not in sys.modules
+
+        canonical_upgrade = name.replace("odoo.addons.base.maintenance.migrations", "odoo.upgrade")
+
+        if canonical_upgrade in sys.modules:
+            mod = sys.modules[canonical_upgrade]
+        else:
+            mod = importlib.import_module(canonical_upgrade)
+
+        sys.modules[name] = mod
+
+        return sys.modules[name]
+
+
 def initialize_sys_path():
     """
     Setup an import-hook to be able to import OpenERP addons from the different
@@ -142,6 +169,7 @@ def initialize_sys_path():
     sys.modules["odoo.addons.base.maintenance.migrations"] = upgrade
 
     if not getattr(initialize_sys_path, 'called', False): # only initialize once
+        sys.meta_path.insert(0, UpgradeHook())
         sys.meta_path.insert(0, OdooHook())
         sys.meta_path.insert(0, AddonsHook())
         initialize_sys_path.called = True