Replace UnixGlob's package glob prefetching functionality with an
optional method in FileSystem.  Custom FileSystem implementations can
use this to provide their own implementation of glob prefetching.

--
MOS_MIGRATED_REVID=140736304
diff --git a/src/main/java/com/google/devtools/build/lib/vfs/Path.java b/src/main/java/com/google/devtools/build/lib/vfs/Path.java
index e039efe..cdc0a0a 100644
--- a/src/main/java/com/google/devtools/build/lib/vfs/Path.java
+++ b/src/main/java/com/google/devtools/build/lib/vfs/Path.java
@@ -1227,6 +1227,10 @@
     fileSystem.chmod(this, mode);
   }
 
+  public void prefetchPackageAsync(int maxDirs) {
+    fileSystem.prefetchPackageAsync(this, maxDirs);
+  }
+
   /**
    * Compare Paths of the same file system using their PathFragments.
    *