Extending a class which is created by extension builder is only possible after installing the extension

[simonschaufi]

Describe the bug Extending a class which is created by extension builder is only possible after installing the extension

To Reproduce Steps to reproduce the behavior:

1. Create a class "ParentClass"
2. Create a class "ChildClass"
3. Extend the ParentClass in the ChildClass by enterning the full namespace like Example\Extension\Domain\Model\ParentClass
4. See error

Expected behavior Extending a class should be possible during creating the extension already without installing it first

Affected versions v9 and master