Flags a part of directive that names its library instead of using a string URI.
A part of my.library.name; directive identifies the enclosing library by its
declared name, which forces the parent to carry a library name and couples the
part to it indirectly. The URI form, part of 'parent.dart';, points straight at
the containing file, is more robust, and is the form modern Dart tooling prefers.
The rule reports only the named form (a non-empty dotted name with no string URI).