Support debug output for macros

It's sometimes useful to get the output of the macro-generated code.
This adds a mechanism to allow plugins.sbt to pass in scalacOptions,
which we can later check from the macro.

1. -Xmacro-settings:sbt:Vprint prints out the code.
2. Adding -Xmacro-settings:sbt:print-tree-structure prints out the tree structure.

internal/util-interface/src/main/java/xsbti/Attic.java

@@ -0,0 +1,26 @@
+package xsbti;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+
+/** A global in-memory storage to pass information. */
+public class Attic {
+  private static List<String> _items = new ArrayList<String>();
+
+  /**
+   * This is used to collect scalacOptions of metabuild. This works around the fact that
+   * CompilationInfo.XmacroSettings is experimental.
+   */
+  public static void appendItems(Collection<String> values) {
+    _items.addAll(values);
+  }
+
+  /**
+   * This is used to return scalacOptions of metabuild. This works around the fact that
+   * CompilationInfo.XmacroSettings is experimental.
+   */
+  public static Collection<String> getItems() {
+    return _items;
+  }
+}