Support unnamed libraries

[srawlins]

Description

Support unnamed libraries across all lint rules. The only ones which use LibraryDirective are:

• library_names
• package_prefixed_library_name which is just a big comment. So, easy peasy.

[srawlins]

@bwilkerson as far as tests, since each Dart library can only have one library directive, I think I need to convert the library_names lint rule to use reflective loader tests. Is there a good example PR of that I can follow?

[coveralls]

Coverage increased (+0.0005%) to 95.68% when pulling 16fbf34 on unnamed-2 into cbe7f4e on main.

[bwilkerson]

I don't know of one, but we haven't been requiring converting older tests to the new style; we have several lints with tests of both forms.

[srawlins]

Since each Dart library can only have one library directive, I think I need to convert the library_names lint rule to use reflective loader tests, right?

[srawlins]

I switched to unit tests; so very nice.