Add some utilities and relax some visibility restrictions to make alternative include scanning implementations possible.

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MOS_MIGRATED_REVID=96337469
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/FileValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/FileValue.java
index 1850fd9..db5ed24 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/FileValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/FileValue.java
@@ -50,7 +50,7 @@
 @ThreadSafe
 public abstract class FileValue implements SkyValue {
 
-  boolean exists() {
+  public boolean exists() {
     return realFileStateValue().getType() != Type.NONEXISTENT;
   }