Make some skyframe and lib/skyframe classes public.

--
MOS_MIGRATED_REVID=99197069
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
index 745df28..ab25cd8 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
@@ -26,14 +26,13 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 /** Common utilities for dealing with files outside the package roots. */
-class ExternalFilesHelper {
+public class ExternalFilesHelper {
 
   private final AtomicReference<PathPackageLocator> pkgLocator;
   private final Set<Path> immutableDirs;
   private final boolean errorOnExternalFiles;
 
-  @VisibleForTesting
-  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator) {
+  public ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator) {
     this(pkgLocator, ImmutableSet.<Path>of(), /*errorOnExternalFiles=*/false);
   }