Support `extern alias` in generated code.

I was benchmarking the old and new version of my library and the benchmark methods were returning objects of a type with the same name from the old or new assembly. I can't vary the NuGet package versions because the new version is not yet released and the API is different.

In my benchmark code I used `extern alias` to point to the old library but this did not carry over to the code BenchmarkDotNet generated, causing `error CS0433: The type 'Grammar' exists in both 'Farkle, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' and 'FarkleNeo, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'`.

I found a workaround by returning `object` so I guess it's a pretty niche scenario.