`__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>