* Restore overridden modules, for 2 reasons:
  - The user may choose to not install the new module.
  - Modules in BT may import from ERP5Type.Document the classes they override
    (although I am sure it is good practice).
* Check existence of oid in pickle cache before deleting it, because it may
  have been already removed automatically.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31801 20353a03-c40f-0410-a6d1-a30d3c3de9de