Add ActionInput.getExecPath, which returns a PathFragment

Update some callers to use getExecPath, which generally results in less
intermediate garbage generation (almost all callers need a PathFragment, not
a String).

Another small step towards #1593.

--
PiperOrigin-RevId: 150631279
MOS_MIGRATED_REVID=150631279
diff --git a/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java b/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
index 4ca4a1b..c63fd5b 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
@@ -69,6 +69,7 @@
    */
   private static class BasicActionInput implements ActionInput {
     private final String path;
+
     public BasicActionInput(String path) {
       this.path = Preconditions.checkNotNull(path);
     }
@@ -79,6 +80,11 @@
     }
 
     @Override
+    public PathFragment getExecPath() {
+      return new PathFragment(path);
+    }
+
+    @Override
     public int hashCode() {
       return path.hashCode();
     }