Add an unbranded TCGC entrypoint

It should be possible to define an unbranded version of TCGC by giving the package a different name in the CreateLibrary call. This would allow unbranded emitters to avoid having any azure dependencies.

This came from the fact that there were diagnostics being emitted from TCGC and the unbranded libraries that use the unbranded C# emitter didn't want to reference "Azure" in their specs.

According to @timotheeguerin, we could easily create a separate package that has the same content, not sure if having this separate entry point will add overhead to our release process though. In the past, there have also been talks about adding a new unbranded tcgc package built on top of efv2, so would have to consider this as well.

cc @lmazuel to talk about once you're back from vacation