tool/cmd/migrate: dotnet: get rid of package groups

I'd like us to get rid of package groups entirely... if we don't, I think we'll need to think carefully about how they're represented in Librarian. (In v0.1.0, each package group is a library - the libraries within it *aren't* Librarian-libraries. It's messy, as you can tell...)

_Originally posted by @jskeet in https://github.com/googleapis/librarian/pull/4414#discussion_r2904695095_

Start in tool/cmd/migrate and trace how package groups are represented and processed for .NET. Determine the complete scope needed to remove package groups, then verify that migration no longer treats groups as Librarian libraries; the issue names no tests or other files to run.

Written by the indexing model from the issue text.