Change the key type we keep around in our fileset mappings.

There is no need to throw away the fact that the mapping is artifact -> fileset symlinks so early.

This CL is otherwise a no-op.

RELNOTES: None
PiperOrigin-RevId: 211808154
diff --git a/src/main/java/com/google/devtools/build/lib/vfs/OutputService.java b/src/main/java/com/google/devtools/build/lib/vfs/OutputService.java
index 0f87039..d45912d 100644
--- a/src/main/java/com/google/devtools/build/lib/vfs/OutputService.java
+++ b/src/main/java/com/google/devtools/build/lib/vfs/OutputService.java
@@ -143,8 +143,10 @@
    * @param filesets The Fileset symlinks known for this action.
    */
   default void updateActionFileSystemContext(
-      FileSystem actionFileSystem, Environment env, MetadataConsumer consumer,
-      ImmutableMap<PathFragment, ImmutableList<FilesetOutputSymlink>> filesets)
+      FileSystem actionFileSystem,
+      Environment env,
+      MetadataConsumer consumer,
+      ImmutableMap<Artifact, ImmutableList<FilesetOutputSymlink>> filesets)
       throws IOException {}
 
   default boolean supportsPathResolverForArtifactValues() {