Share full classpath NestedSet between full and header compile actions

The Starlark API already adds the direct JARs to the transitive
classpath `depset`, so the deleted code path didn't actually change the
set. However, it resulted in separate instances being passed to and
retained by `JavaCompileAction` and `JavaHeaderCompileAction` in the
case where the direct classpath optimization isn't used.

src/main/java/com/google/devtools/build/lib/rules/java/JavaTargetAttributes.java

@@ -321,16 +321,10 @@ public Builder addAdditionalOutputs(Iterable<Artifact> outputs) {
 
     public JavaTargetAttributes build() {
       built = true;
-      NestedSet<Artifact> directJars = directJarsBuilder.build();
-      NestedSet<Artifact> compileTimeClassPath =
-          NestedSetBuilder.<Artifact>naiveLinkOrder()
-              .addTransitive(directJars)
-              .addTransitive(compileTimeClassPathBuilder.build())
-              .build();
       return new JavaTargetAttributes(
           ImmutableSet.copyOf(sourceFiles),
           runtimeClassPath.build(),
-          compileTimeClassPath,
+          compileTimeClassPathBuilder.build(),
           bootClassPath,
           sourcePath,
           nativeLibraries.build(),
@@ -341,7 +335,7 @@ public JavaTargetAttributes build() {
           ImmutableList.copyOf(sourceJars),
           classPathResources.build(),
           additionalOutputs.build(),
-          directJars,
+          directJarsBuilder.build(),
           compileTimeDependencyArtifacts.build(),
           targetLabel,
           injectingRuleKind,