CD compiler does not report unresolved imported types

[schmalzing]

Issue description

When compiling a MontiCore class diagram, an invalid import is accepted without an error. The problem is only detected later when the generated Java code is compiled.

Reproduction

montiarc.generator.meta.Java.cd:

classdiagram Java {
  public abstract class Class { }
}

montiarc.generator.meta.MA2Jsim.cd:

import montiarc.generator.Java.Class;

classdiagram MA2Jsim {
  public class MCConcept extends Class { }
}

The import in MA2Jsim.cd is invalid. The declared type is actually:

montiarc.generator.meta.Java.Class

Actual behavior

The CD compilation succeeds and generates Java code containing:

import montiarc.generator.Java.Class;

Java compilation then fails, for example:

ERROR in ...\MA2Jsim\Transition.java (at line 21)
import montiarc.generator.Java.Class;
       ^^^^^^^^^^^^^^^^^^^^^^^
The import montiarc.generator.Java cannot be resolved

Expected behavior

The CD compiler should report an error during CD processing because montiarc.generator.Java.Class cannot be resolved.

The correct import is:

import montiarc.generator.meta.Java.Class;

Impact

Invalid cross-CD dependencies remain undetected until the Java-generation/Java-compilation phase. This delays feedback and makes the error appear as a generated-code or Java classpath problem rather than an invalid CD import.

[luepges]

-> a CD4Code CoCo (that is added by the generator) should catch this