Introduce the 'siblings' query function.

RELNOTES[NEW]: There is now a 'siblings' query function. See the query documentation for more details.

PiperOrigin-RevId: 165010653
diff --git a/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
index 84313ae..a9130f7 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
@@ -691,6 +691,12 @@
 
   @ThreadSafe
   @Override
+  public Collection<Target> getSiblingTargetsInPackage(Target target) {
+    return target.getPackage().getTargets().values();
+  }
+
+  @ThreadSafe
+  @Override
   public QueryTaskFuture<Void> getTargetsMatchingPattern(
       QueryExpression owner, String pattern, Callback<Target> callback) {
     TargetPatternKey targetPatternKey;