fix: compute longest common prefix for sibling packages in empty source roots

When a Bazel package contains source files from sibling Java packages
(e.g. dto/ and merge/) without files in the root package,
findCommonParentPackagePrefix returned null because it only checked
if a detected package was a prefix of all others. This caused
"an empty package fragment root must map to one Java package" errors.

Now falls back to computing the actual longest common prefix path,
allowing targets like messier_merger_lib with sources spanning
sibling sub-packages to be provisioned correctly.

Also adds support for merging multiple targets sharing an empty source
root into a single Eclipse project, and collects dependencies from
unprovisioned sibling targets for complete classpath computation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: runchen0919

bundles/com.salesforce.bazel.eclipse.core/src/com/salesforce/bazel/eclipse/core/model/discovery/BaseProvisioningStrategy.java

@@ -1262,7 +1262,28 @@ private IPath findCommonParentPackagePrefix(Collection<IPath> detectedJavaPackag
             }
         }
 
-        return null;
+        // none of the detected packages is a prefix of all others (sibling packages case)
+        // compute the actual longest common prefix path
+        // e.g. for [com/foo/bar/dto, com/foo/bar/merge] this yields com/foo/bar
+        IPath commonPrefix = null;
+        for (IPath path : detectedJavaPackagesForSourceDirectory) {
+            if (commonPrefix == null) {
+                commonPrefix = path;
+            } else {
+                var minLen = Math.min(commonPrefix.segmentCount(), path.segmentCount());
+                var commonLen = 0;
+                for (var i = 0; i < minLen; i++) {
+                    if (commonPrefix.segment(i).equals(path.segment(i))) {
+                        commonLen = i + 1;
+                    } else {
+                        break;
+                    }
+                }
+                commonPrefix = commonPrefix.uptoSegment(commonLen);
+            }
+        }
+
+        return (commonPrefix == null) || commonPrefix.isEmpty() ? null : commonPrefix;
     }
 
     private IProject findProjectForLocation(IPath location) {