Move classes needed by AbstractBuilderState from ui to core plug-in

The class AbstractBuilderState uses these three classes which are defined in an UI plugin,
import org.eclipse.xtext.ui.resource.IStorage2UriMapper;
import org.eclipse.xtext.ui.resource.IStorage2UriMapperExtension;
import org.eclipse.xtext.ui.workspace.WorkspaceLockAccess;

all of this classes reference only classes from org.eclipse.core (no UI references).

We have subclassed org.eclipse.xtext.builder.clustering.ClusteringBuilderState and because of these three references we need to include org.eclipse.xtext.ui as dependency, which we would like to avoid.

I would propose that we create three new classes
org.eclipse.xtext.resource.IStorage2UriMapper;
org.eclipse.xtext.resource.IStorage2UriMapperExtension;
org.eclipse.xtext.workspace.WorkspaceLockAccess;
with exactly the same content but in the org.eclipse.xtext plug-in.
The three existing classes would just extend the new ones (same pattern as with org.eclipse.core.resources.IWorkspaceRunnable and org.eclipse.core.runtime.ICoreRunnable.

If you would agree with the changes, I would like to do them. Maybe it does not have much benefits for Xtext code base but it would be nice for us and the changes are easy to do and to mantain.

What do you think?

Start by locating AbstractBuilderState and the three referenced UI classes: IStorage2UriMapper, IStorage2UriMapperExtension, and WorkspaceLockAccess. Review how the existing core/UI interface pattern is organized, then verify the proposed core-package classes and UI compatibility layer can remove the UI dependency without breaking existing references.