Refactor TopLevel tree node.

Currently a TopLevel tree node has explicit members for the optional package name and any annos on it. A better impl would be to have a PackageDef node that bundles package name, doc comment and annos, and have PackageDef live in the List<JCTree> defs in the TopLevel. Some dancing would be required for interface compatibility in com.sun.source.tree.CompilationUnit, maybe using defauilt methods.