Now that SkyKey is an interface, accept Iterable<? extends SkyKey>

A subsequent CL makes TargetPatternKey implement SkyKey, and it's much nicer
if I can pass lists of TargetPatternKey to the various Skyframe APIs.

PiperOrigin-RevId: 162359843
diff --git a/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java b/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
index 07f8faf..26422dd 100644
--- a/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
+++ b/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
@@ -46,7 +46,8 @@
    * @param reason the reason the nodes are being requested.
    */
   Map<SkyKey, ? extends NodeEntry> getBatch(
-      @Nullable SkyKey requestor, Reason reason, Iterable<SkyKey> keys) throws InterruptedException;
+      @Nullable SkyKey requestor, Reason reason, Iterable<? extends SkyKey> keys)
+          throws InterruptedException;
 
   /**
    * Examines all the given keys. Returns an iterable of keys whose corresponding nodes are