Refactor Routing to use ProjectReference and move into src/Http folder

[natemcmaster]

This implements #4246 for projects that were under src/Routing.

Changes:

• Stop producing Microsoft.AspNetCore.Routing.DecisionTree.Sources. I could not find any usages of outside of the Microsoft.AspNetCore.Routing assembly, so I've moved this to be a subfolder of code in the Microsoft.AspNetCore.Routing project.
• Stop building src/Routing as a submodule. Use ProjectReference instead
• Move projects from src/Routing into src/Http/Routing/ and src/Http/Routing.Abstractions/
• Update package baselines and dependencies to support building routing projects and samples

[natemcmaster]

Two test failures in HttpSys: FYI @jkotalik @muratg

• Microsoft.AspNetCore.Server.HttpSys.FunctionalTests.Microsoft.AspNetCore.Server.HttpSys.Listener.ResponseCachingTests.Caching_SendFileWithFullContentLength_Cached
• Microsoft.AspNetCore.Server.HttpSys.FunctionalTests.Microsoft.AspNetCore.Server.HttpSys.ServerTests.Server_MultipleStopAsyncCallsWaitForRequestsToDrain_Success

These appear to be unrelated to Routing.

[Tratcher]

I'm torn between putting this in Http vs Middleware. @JamesNK @rynowak?

[JamesNK]

While routing has middleware, its job to match an HTTP request to an endpoint does feel more fundamental than other middleware. Plus routing has link generation, and that isn't middleware specific. I prefer Http.

[natemcmaster]

If there are no more objections, I'd like to merge this later today.