DocFX currently only supports generating metadata with namespace defined

[jwittner]

I have a large code base with many types without a namespace. I understand from #417 that this is viewed as bad form, but I cannot change this about the structure of the code at this point.

I'm absolutely willing to add a solution for this, but would love to get feedback on the recommended change to support this. At a quick glance it looks like I could traverse the members of the global namespace in SymbolVisitorAdapter.cs instead of logging the warning.