Add package declaration node to synthetic ASTs

When there is a big syntactic problem with a file, the compiler gives us back a synthetic AST that has minimal information. It would be helpful if this AST contained a package declaration as well, because the absence of it causes X10DT's package declaration check to fail. This is bad because the editor shows package declaration errors even when it is correct, when there are bad syntactic problems. When the user fixes the syntax problems, the package declaration error goes away on its own. So this looks very bad for the editor. If we get back a synthetic AST that has the package declaration then we would not have this problem.