Add support for resolving target platform projects

Currently target-platform projects do not have any resolved
dependencies, this has a few disadvantages:

- if one want to see the dependency tree it is empty
- if one wants to mirror a target platform this can't be performed

This adds support for target platform projects to resolve and have their
dependencies injected to support such advanced use-cases.

tycho-core/src/main/java/org/eclipse/tycho/core/osgitools/TargetPlatformProject.java

@@ -0,0 +1,79 @@
+/*******************************************************************************
+ * Copyright (c) 2023 Christoph Läubrich and others.
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *    Christoph Läubrich - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.tycho.core.osgitools;
+
+import java.io.File;
+
+import org.codehaus.plexus.component.annotations.Component;
+import org.eclipse.tycho.ArtifactKey;
+import org.eclipse.tycho.DefaultArtifactKey;
+import org.eclipse.tycho.DependencyArtifacts;
+import org.eclipse.tycho.PackagingType;
+import org.eclipse.tycho.ReactorProject;
+import org.eclipse.tycho.TychoConstants;
+import org.eclipse.tycho.core.ArtifactDependencyVisitor;
+import org.eclipse.tycho.core.ArtifactDependencyWalker;
+import org.eclipse.tycho.core.TychoProject;
+import org.eclipse.tycho.core.osgitools.targetplatform.DefaultDependencyArtifacts;
+import org.eclipse.tycho.model.Feature;
+import org.eclipse.tycho.model.ProductConfiguration;
+import org.eclipse.tycho.model.UpdateSite;
+
+@Component(role = TychoProject.class, hint = PackagingType.TYPE_ECLIPSE_TARGET_DEFINITION)
+public class TargetPlatformProject extends AbstractTychoProject {
+
+    @Override
+    public ArtifactDependencyWalker getDependencyWalker(ReactorProject project) {
+        return new ArtifactDependencyWalker() {
+
+            @Override
+            public void walk(ArtifactDependencyVisitor visitor) {
+
+            }
+
+            @Override
+            public void traverseUpdateSite(UpdateSite site, ArtifactDependencyVisitor visitor) {
+
+            }
+
+            @Override
+            public void traverseProduct(ProductConfiguration productConfiguration, ArtifactDependencyVisitor visitor) {
+
+            }
+
+            @Override
+            public void traverseFeature(File location, Feature feature, ArtifactDependencyVisitor visitor) {
+
+            }
+        };
+    }
+
+    @Override
+    public ArtifactKey getArtifactKey(ReactorProject project) {
+        return new DefaultArtifactKey("target", project.getArtifactId(), project.getVersion());
+    }
+
+    @Override
+    public DependencyArtifacts getDependencyArtifacts(ReactorProject project) {
+        synchronized (project) {
+            DependencyArtifacts resolvedDependencies = (DependencyArtifacts) project
+                    .getContextValue(TychoConstants.CTX_DEPENDENCY_ARTIFACTS);
+            if (resolvedDependencies != null) {
+                return resolvedDependencies;
+            }
+            DefaultDependencyArtifacts artifacts = new DefaultDependencyArtifacts(project);
+            return artifacts;
+        }
+    }
+
+}