Improve imported package validation

[ZdenekM]

When importing an execution package, the Build service currently, for each object type in scene.json, tries to:

• Parse its code (to check if the syntax is valid Python).
• Import the ObjectType's class definition (to check whether all imports work and all ancestors are available).

Additionally, it should:

• Raise an error if ObjectType.abstract() is True (abstract type can't be used in a scene as it has undefined/abstract methods and/or is marked as abstract manually/intentionally).
• ??

[martincervinka]

Is this issue somehow related to #690? @ZdenekM, can you explain it, please?

[ZdenekM]

@martincervinka It is a (partial) duplicate, I will close it. But that brought me to the question - should we retain the import functionality? Do you use it? Let me know and I will update the issue.