[Bug] Cloning of CtUnresolvedImport does not work

**Describe the bug**
When cloning a compilation unit `CtUnresolvedImport` is turned into a `CtImportImpl` with no reference set. The unresolved reference is lost in translation.

**To Reproduce**
I found that while investigating issue #4593. The issue contains reproduction steps.

**The problem**
When changing the class name, the compilation unit is cloned in https://github.com/INRIA/spoon/blob/8397c32d7aa3f5389a21148b506e1b2c6c5938c6/src/main/java/spoon/reflect/visitor/DefaultJavaPrettyPrinter.java#L2149

This clone ends up in the clone method for `CtImport`:
```java
// auto-generated, see spoon.generating.CloneVisitorGenerator
@java.lang.Override
public void visitCtImport(final spoon.reflect.declaration.CtImport ctImport) {
spoon.reflect.declaration.CtImport aCtImport = ctImport.getFactory().Core().createImport();
this.builder.copy(ctImport, aCtImport);
aCtImport.setReference(this.cloneHelper.clone(ctImport.getReference()));
aCtImport.setAnnotations(this.cloneHelper.clone(ctImport.getAnnotations()));
aCtImport.setComments(this.cloneHelper.clone(ctImport.getComments()));
this.cloneHelper.tailor(ctImport, aCtImport);
this.other = aCtImport;
}
```
As you can see, this always creates a new `CtImportImpl`, but never a `CtUnresolvedImport`. It also never copies the `String unresolvedReference` of the unresolved import. This is bad.

**Possible solutions**
1. Make the CtUnresolvedImport a metamodel element. If we do this we need to rename the class and introduce an interface for it.
2. Special-case the visitCtImport method in the clone visitor generator and hard code an `instanceof` distinction in there. This doesn't really sound appealing.

The first option sounds cleaner but is breaking (and maybe complicates the existence of the CtImportVisitor a bit?). Imports are currently in the `experimental` package, so maybe this is a worthwhile change.

**Moving forward**
I could probably implement both solutions but I'd like some input from the integrators to decide which option to proceed with.