speed up sourcelink test

[ctaggart]

When dotnet sourcelink test is currently run, it downloads all of the source files sequentially and synchronously. This could be made asynchronous and parallel.

Current code:

                foreach (var doc in GetDocumentsWithUrls(drp))
                {
                    if(doc.Url != null)
                    {
                        HashUrl(hc, doc);

[mderriey]

Hey @ctaggart I'm keen on giving this a go.

Have you already thought about how you'd like this to be implemented?

[ctaggart]

@mderriey I have not. Give it a go. There may be multiple files in a nupkg to check urls for.

[mderriey]

OK, cool.