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/ParallelEvaluator.java b/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluator.java
index aac9277..a47a579 100644
--- a/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluator.java
+++ b/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluator.java
@@ -690,7 +690,7 @@
 
   @Override
   @ThreadCompatible
-  public <T extends SkyValue> EvaluationResult<T> eval(Iterable<SkyKey> skyKeys)
+  public <T extends SkyValue> EvaluationResult<T> eval(Iterable<? extends SkyKey> skyKeys)
       throws InterruptedException {
     ImmutableSet<SkyKey> skyKeySet = ImmutableSet.copyOf(skyKeys);