Fix class name resolution of prototype runtime

[ksss]

In $ rbs prototype runtime, there was an unintended output in the case of class.

---

t.rb

module Foo
  class Bar
    class Baz
    end
  end
end

before

$ bundle exec rbs prototype runtime --require-relative t.rb 'Foo::*'
module Foo
  class Bar
  end

  module Bar
    class Baz
    end
  end
end

$ bundle exec rbs -r pathname -r singleton -r tsort prototype runtime -R lib/rbs 'RBS::*' > before.rbs

$ bundle exec rbs -r pathname -r singleton -r tsort -I before.rbs validate --silent
bundler: failed to load command: rbs (/Users/ksss/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/bin/rbs)
/Users/ksss/src/github.com/ksss/rbs/lib/rbs/environment.rb:190:in `insert_decl': before:582:2...610:5: Duplicated declaration: ::RBS::AncestorGraph (RBS::DuplicatedDeclarationError)

after

$ bundle exec rbs prototype runtime --require-relative t.rb 'Foo::*'
module Foo
  class Bar
    class Baz
    end
  end
end

$ bundle exec rbs -r pathname -r singleton -r tsort prototype runtime -R lib/rbs 'RBS::*' > after.rbs

$ bundle exec rbs -r pathname -r singleton -r tsort -I after.rbs validate --silent
# no error

[pocke]

Thanks!