[IMP] core: formally deprecate __openerp__.py manifests.

`__manifest__.py` was introduced in Odoo 10 and quickly migrated
to (c758e9043a fixed the last holdouts).

Deprecate old manifests. People who need the compatibility can just
add a symlink (or duplicate the file if they're working on FAT or an
old os where core.symlinks can't be set to `true`).

closes odoo/odoo#103952

Signed-off-by: Raphael Collet <rco@odoo.com>

odoo/modules/module.py

@@ -316,8 +316,16 @@ def module_manifest(path):
     if not path:
         return None
     for manifest_name in MANIFEST_NAMES:
-        if os.path.isfile(opj(path, manifest_name)):
-            return opj(path, manifest_name)
+        candidate = opj(path, manifest_name)
+        if os.path.isfile(candidate):
+            if manifest_name == '__openerp__.py':
+                warnings.warn(
+                    "__openerp__.py manifests are deprecated since 17.0, "
+                    f"rename {candidate!r} to __manifest__.py "
+                    "(valid since 10.0)",
+                    category=DeprecationWarning
+                )
+            return candidate
 
 def get_module_root(path):
     """