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/InMemoryGraph.java b/src/main/java/com/google/devtools/build/skyframe/InMemoryGraph.java
index 267ae78..e8c7d35 100644
--- a/src/main/java/com/google/devtools/build/skyframe/InMemoryGraph.java
+++ b/src/main/java/com/google/devtools/build/skyframe/InMemoryGraph.java
@@ -28,7 +28,7 @@
 
   @Override
   Map<SkyKey, ? extends NodeEntry> getBatch(
-      @Nullable SkyKey requestor, Reason reason, Iterable<SkyKey> keys);
+      @Nullable SkyKey requestor, Reason reason, Iterable<? extends SkyKey> keys);
 
   /**
    * Returns a read-only live view of the nodes in the graph. All node are included. Dirty values