Index import, export, part, and part of directives

[matthewnitschke-wk]

Imports and their prefixes are completely ignored, these should be indexed so they can be navigated to

import 'dart:math' as math;
import 'package:some_package/package.dart';
import '../foo/bar.dart' as something;
import '../other.dart' hide Foo;
import '../other2.dart' show Bar;
export 'somewhere.dart';
part 'somewhere.dart';
part of '../somewhere_else.dart';