Unregistering a ConnectorUnit from a backend unregisters even when the addon is not installed

[Guewen Baconnier @ Camptocamp]

Example:

    # 7.0-magentoerpconnect : /magentoerpconnect_pricing/product.py
    magento.unregister_class(product.ProductImportMapper)

    [...]
    @magento
    class ProductImportMapper(product.ProductImportMapper):
        _model_name = 'magento.product.product'

        @only_create
        @mapping
        def price(self, record):
            """ The price is imported at the creation of
            the product, then it is only modified and exported
            from OpenERP """
            return super(ProductImportMapper, self).price(record)

Expected:

When the adddon magentoerpconnect_pricing is installed, the magentoerpconnect's ProductImportMapper is unregistered and the magentoerpconnect_pricing's ProductImportMapper replace it.

Current behavior:

Even if magentoerpconnect_pricing is not installed, magentoerpconnect's ProductImportMapper is unregistered and replaced by the magentoerpconnect_pricing's ProductImportMapper. This is due to the fact that OpenERP import the Python module of all the addons, whether they are installed or not.

Correction proposition:

Remove the ``unregister`` capabilities on the backends.
Add a ``replacing`` when a ConnectorUnit is registered on a backend.

    [...]
    @magento(replacing=product.ProductImportMapper)
    class ProductImportMapper(product.ProductImportMapper):

The ConnectorUnit is determined at runtime, with an algorithm as follows:

    for each ConnectorUnit registered on the backend
        Exclude it if the addons where it is declared is not installed -> if the ConnectorUnit is replaced by another (could be several if many addons) -> append to the candidate list (recursively)
        returns the last candidate or an error if many / 0 are found

[Guewen Baconnier @ Camptocamp (gbaconnier-c2c)]

Not sure if this really happens, I couldn't reproduce it.
However, strongly related to: lp:1187697

[Guewen Baconnier @ Camptocamp (gbaconnier-c2c)]

Released in

https://bazaar.launchpad.net/~openerp-connector-core-editors/openerp-connector/7.0-magentoerpconnect/revision/907
https://bazaar.launchpad.net/~openerp-connector-core-editors/openerp-connector/7.0/revision/583