Sourcelink for code generators

[daveMueller]

closes #1322

[daveMueller]

While analyzing #1322 I stumbled across the issue that getting the sourcelink url for a code generator class throws. As a quick fix I just returned the relative path so that it doesn't fail anymore. But I could also think of something like removing those files from the coverage result?

I think there is still a general problem with code generators. Sure we can collect hits et cetera, but we can't really map this to a source file because initially no physical file exists. And I think there are a couple of places in coverlet where this causes problems. E.g. the SourceRootTranslator can't resolve the file path because it doesn't exist.
Maybe it would be the best to exclude those files from the instrumentation? Was there already a discussion about this that I missed?
cc: @MarcoRossignoli @petli @tonerdo

[MarcoRossignoli]

I think that at the moment we have this one #1164 on the subject