[10.0][MIG][base_import_match] Migration and update

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.

base_import_match/models/__init__.py

@@ -3,4 +3,5 @@
 # Copyright 2016 Tecnativa - Vicent Cubells
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
+from . import base
 from . import base_import