Add syntactic sugar for package imports

In Dart, it's extremely common to see imports that look like this

```dart
import 'package:epubx/epubx.dart';
import 'package:path/path.dart' as path;
import 'package:quiver/collection.dart';
import 'package:web/web.dart';
import 'package:xml/xml.dart';
```

The repetition in there should be plain to see. As it is the case that it's standard for libraries to put the primarily intended import in a file with the same name as the package, I suggest adding syntactic sugar where `import 'package:foo'` would be treated the same as `import 'package:foo/foo.dart'`. Under this proposed change, the current syntax would remain valid.

With this feature, the example above would instead look like

```dart
import 'package:epubx';
import 'package:path' as path;
import 'package:quiver/collection.dart';
import 'package:web';
import 'package:xml';
```

Beyond being less typing and less reading, and generally looking cleaner, I believe this sugar would also make it easier to see at a glance which packages are being imported partially. It would also allow these "standard" package imports to better mirror the format of Dart SDK imports (`import 'dart:async;'`).