Fix rbuildfiles query operation for broken packages. We use to assume that all the packages referenced by subinclude files should be in the graph. But this is not the case when the package is in error (The package is in the graph but as an error value, not as a package value). This produced the crash seen in [1] for a simple query like rbuildfiles(broken/BUILD).

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MOS_MIGRATED_REVID=102869135
diff --git a/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java b/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
index cd87633..a8c18ea 100644
--- a/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
+++ b/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
@@ -41,10 +41,11 @@
   SkyValue getValue(SkyKey key);
 
   /**
-   * Returns a map giving the values of the given keys for done keys. Keys not present in the graph
-   * or whose nodes are not done will not be present in the returned map.
+   * Returns a map giving the values of the given keys for done keys that were successfully
+   * computed. Or in other words, it filters out non-existent nodes, pending nodes and nodes
+   * that produced an exception.
    */
-  Map<SkyKey, SkyValue> getDoneValues(Iterable<SkyKey> keys);
+  Map<SkyKey, SkyValue> getSuccessfulValues(Iterable<SkyKey> keys);
 
   /**
    * Returns a map giving exceptions associated to the given keys for done keys. Keys not present in