package_names 
            
            Use lowercase_with_underscores for package names.
This rule is available as of Dart 2.0.0.
Rule sets: recommended, flutter
Details
From the Pubspec format description:
DO use lowercase_with_underscores for package names.
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.
Usage
To enable the package_names rule,
add package_names under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - package_names