Prevent vendor class name overlap

[pkly]

Fixes #1566

I've pasted this over the currently installed version in the project in which I had these issues, seems to work fine. Would be good if it could be re-checked and generally looked over, since I'm not 100% sure about it, but this is the place which causes the issue mentioned.

It should prevent situations where a known, working class name is skipped over and a vendor class is used instead, as there is no instance in which modifying vendor classes is required.

Actually this also seems to not work when it comes to classes with relations, so the target entity class (one we're adding relation to) may still be pulled from the vendor.