Add static methods to return target kinds for built-in targets

Add static methods to return target kinds of InputFile and OutFile, as well as
suffix of target kind of Rule.

RELNOTES: None
PiperOrigin-RevId: 154866926
diff --git a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
index 6f2fe22..c04a346 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
@@ -109,7 +109,7 @@
 
   @Override
   public String getTargetKind() {
-    return "source file";
+    return targetKind();
   }
 
   @Override
@@ -121,4 +121,9 @@
   public Location getLocation() {
     return location;
   }
+
+  /** Returns the target kind for all input files. */
+  public static String targetKind() {
+    return "source file";
+  }
 }