Support source code lookup for classes not on the project classpath

bundles/com.salesforce.bazel.eclipse.core/plugin.xml

@@ -158,6 +158,12 @@
             id="com.salesforce.bazel.eclipse.core.classpath.launching.runtimeResolver">
       </runtimeClasspathEntryResolver>
    </extension>
+    <extension
+          point="org.eclipse.jdt.launching.sourceContainerResolvers">
+       <resolver
+             class="com.salesforce.bazel.eclipse.core.classpath.BazelSourceContainerResolver">
+       </resolver>
+    </extension>
 
     <extension
           point="org.eclipse.core.contenttype.contentTypes">

bundles/com.salesforce.bazel.eclipse.core/src/com/salesforce/bazel/eclipse/core/classpath/BazelSourceContainerResolver.java

@@ -0,0 +1,45 @@
+/*-
+ * Copyright (c) 2023 Salesforce and others.
+ *
+ * All rights reserved. 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:
+ *      Salesforce - adapted from M2E, JDT or other Eclipse project
+ */
+package com.salesforce.bazel.eclipse.core.classpath;
+
+import java.io.File;
+import java.util.Collection;
+import java.util.Set;
+
+import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.IProgressMonitor;
+import org.eclipse.debug.core.sourcelookup.ISourceContainer;
+import org.eclipse.debug.core.sourcelookup.containers.ExternalArchiveSourceContainer;
+import org.eclipse.jdt.launching.sourcelookup.advanced.ISourceContainerResolver;
+
+import com.salesforce.bazel.eclipse.core.util.jar.SourceJarFinder;
+
+/**
+ * A resolver to find source code for Bazel classpath container entries
+ */
+public class BazelSourceContainerResolver implements ISourceContainerResolver {
+
+    @Override
+    public Collection<ISourceContainer> resolveSourceContainers(File classesLocation, IProgressMonitor monitor)
+            throws CoreException {
+
+        var sourceJar = SourceJarFinder.findSourceJar(classesLocation.toPath());
+        if (sourceJar != null) {
+            return Set.of(new ExternalArchiveSourceContainer(sourceJar.toOSString(), true));
+        }
+
+        return null;
+    }
+
+}