[FIX] mrp: prevent editing of readonly locations for MO moves

Issue: In the MO view, the src location for components was editable and
the dest location for byproducts was editable when neither of these
should be.

Due to the same field appearing twice in the same x2many field list,
there were 2 issues:

1. something changed (probably during OWL refactoring) that made it so
   the 2nd invisible field instance of the field was overriding the
   "readonly='1'" property of the 1st instance when it was in the view
   (i.e. when multi-locations is active) [in previous versions this did
   not happen]
2. because of the change by https://github.com/odoo/odoo/commit/168cbe66bee7824bdf389de5c6c680342e27bc6d


   we ensure that these two required fields are always correctly set (to
   the MO's values as per the default when multi-loc is active) when the
   MO's moves are created.

Part of general bugfix task: 2985735

closes odoo/odoo#105959

Related: odoo/enterprise#34276
Signed-off-by: William Henrotin (whe) <whe@odoo.com>