Fix possible double import of cubes For a new-style cube, if we import "cubicweb_<name>" and then import "cubes.<name>", the cube will be imported twice. cubes.<name> and then cubicweb_name is ok though... When using pyramid, we try to find which cube define a "includeme" by importing them with the name "cubes.<name>", so we (possibly ?) double import all new-style cube. This case may also occur in tests with PyramidCWTest. Touching the import loader to fix this scares me, so let's fix this by testing "cubicweb_<name>" *before* "cubes.name" (for old style cubes, importing cubicweb_<name> raise ImportError).

[autoform] Make RelationField.fromcardinality a class method factory Instead of having the "RelationField" class hard-coded in this factory method (thus preventing any subclass to properly use it), we make that method a classmethod and instantiate the actual class instead of the base one.

[autoform] Use fieldclass from kwargs in guess_field for non-final relations This follows up on af969080e7e6 in which a "fieldclass" kwarg has been added in EntityFieldsForm.field_by_name() before calling guess_field(). In this changeset, this value was only used for final relation but the way non-final relation actually also changed since we also use guess_field from these relation (previously, the field class specified through uicfg was instantiated directly in EntityFieldsForm.field_by_name()). So in the non-final case in guess_field() we simply try to retrieve "fieldclass" from kwargs and fall back to RelationField.