Add generate shared framework manifest target, inverse xunit analyzer, reference Test SDK correctly

[ViktorHofer]

Changes:

• Add a generate shared framework manifest file target to handle conflict resolution.
• PackageReference Microsoft.Net.Test.Sdk correctly
• Simplify GenerateRuntimeConfigurationFiles and OverwriteTestRunnerConfigFile logic.
• Exclude xunit.console.runtimeconfig.json from the nuget package to avoid races and duplicate file writes.
• Remove GenerateRunScriptDependsOn as a sequenced extension point isn't needed for that target.
• Remove runtimeconfig.dev.json copying as this file isn't needed anymore on netcoreapp3.0.
• Change RemoveXUnitAnalyzer condition from an opt-in to an opt-out.

[ViktorHofer]

@natemcmaster as you are composing your own shared framework in aspnetcore, you might be interested in commit nr 2 and discussion in dotnet/sdk#3335. If aspnetcore is hitting similar issues we might want to factor our the generate shared deps file task + this new task into its own package.

[natemcmaster]

I'm on parental leave, so not currently working on anything. I would recommend talking with @dagood about shared framework work. Before I started leave, we started working on a way to commonize build tasks and targets for generating a shared framework. See #2704

[ericstj]

Looks reasonable, make sure you have it passing and address feedback in the corefx staging PR before merging.