Consider renaming generated library name to match the package name

[brianquinlan]

For example, right now we have:
google_cloud_ai_generativelanguage_v1beta/lib/generativelanguage.dart

Pub would be happier if the name were:
google_cloud_ai_generativelanguage_v1beta/lib/google_cloud_ai_generativelanguage_v1beta.dart