prefer_relative_imports

Use relative imports for files in the 'lib' directory.

The analyzer produces this diagnostic when an import in a library inside the lib directory uses a package: URI to refer to another library in the same package.

The following code produces this diagnostic because it uses a package: URI when a relative URI could have been used:

import 'package:my_package/bar.dart';

Use a relative URI to import the library:

import 'bar.dart';