[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#71323

X-original-commit: b2ea37f185bc18ecc3701a4f1f1f7a9d37a23078
Signed-off-by: Christophe Simonis <chs@odoo.com>
Co-authored-by: Christophe Simonis <chs@odoo.com>

0 files changed, 0 insertions, 0 deletions