The original code did improperly handle importing
related one2many records where at best all their
attribute values would get overwritten by last
of them. This was caused by mapping them only by
their first part.
Includes:
- Normal migration steps.
- Usage of brand new `_inherit = "base"` in Odoo 10, which implies
removing a lot of monkey-patching code.
- Log a warning when multiple matches are found.
