Lint for pub package names.

[pq]

DO use lowercase_with_underscores for package names.

From the Pubspec format description:

Package names should be all lowercase, with underscores to separate words, just_like_this. Use only basic Latin letters and Arabic digits: [a-z0-9_]. Also, make sure the name is a valid Dart identifier—that it doesn’t start with digits and isn’t a reserved word.

[pq]

Implemented in 1334fd4.