Provide source with nuget using SourceLink

[angularsen]

https://github.com/dotnet/sourcelink

I haven't looked much into it yet, but I assume this replaces the previous solution of shipping a symbols package to SymbolSource.

Update 2018-09-30: Good article on producing and consuming nugets with sourcelink
https://www.hanselman.com/blog/ExploringNETCoresSourceLinkSteppingIntoTheSourceCodeOfNuGetPackagesYouDontOwn.aspx

[angularsen]

@tmilnthorp I reverted the sourcelink PR #538 in master. Let's revisit with your comment in mind:

Either that or we can add them back and use the following to clean up the GitHub diffs: https://robots.thoughtbot.com/github-diff-supression

This would be nice too so you don't have to rerun generate every time you check out, switch branches, etc.

I think this sounds like an interesting idea, and I agree it's annoying you have to generate source locally when switching branches. We could do it as a pre-build step, but that would increase local build times significantly.

I was contemplating complex scenarios using git submodules to link in generated code from other repos, but it just gets real messy and complicated and a lot worse than just reviewing huge diffs on PRs. It also would probably not be compatible with SourceLink either.

If we can use .gitattributes to lazy load/defer diffs of the majority of generated files, then I think that will help a lot. I also think we felt the pain a lot more than normal in v4 branch because we were changing how we generated and structured our code dramatically at the time.

[angularsen]

This was added in v4 in #548 after a bit back and forth.