[WIP] Refactor: Tool now self-contained, BuildTime merged into it

[amis92]

This is a major refactor.

It's triggered by a disaster of what 0.7.5-alpha turned out to be.

Major features:

• dotnet-codegen package is now CodeGeneration.Roslyn.Tool, as per project name.
  This was done to prevent confusion with previous CLI tool, which it now is not at all,
  in terms of either DotNetCliToolReference-style tools and global/local .NET CLI tools.
• Tool is now a DevelopmentDependency=true normal Dependency package (as in,
  suitable for PackageReference usage. Via various properties, it's setup so that it contains
  a framework-dependent executable (.dll) with all dependencies included. Using build/CG.R.Tool.props file, the path to the executable is set in referencing projects.
• BuildTime package's props/targets were merged into the Tool package because a separate package is no longer necessary, and as such necessary consumer dependencies are trimmed down
  to just CodeGeneration.Roslyn.Tool package and a generator plugin of choice.
• Samples added: samples that publish generator packages were added, including one that has an external dependency (Newtonsoft.Json). There's also a build script that uses locally built CG.R packages; that script is also called during GitHub Actions CI workflow.

TODO:

• Update Sdk package as per sample packable generators' csproj files
• Update samples to use Sdk props/targets directly (not via Sdk element).
• Update README
• Update CHANGELOG
• Note down migration steps

To use Sdk package as an Sdk element, the Sdk will need to be published to nuget.org, because MSBuildSdkResolver doesn't work well with local feeds.

[amis92]

@AArnott as seen in https://github.com/AArnott/CodeGeneration.Roslyn/runs/534972275#step:9:12

Nerdbank.GitVersioning v3.1 introduces ExcludeFromCodeCoverage on ThisAssembly,
and the attribute is not available in net20 target from Attributes pkg.

I had to downgrade. I know net20 is not supported, but maybe you could consider doing an exception for it like you did for netstandard1.x in dotnet/Nerdbank.GitVersioning#453 ?

[amis92]

I've updated the wiki for the newest changes: https://github.com/AArnott/CodeGeneration.Roslyn/wiki/Migrations

[amis92]

@AArnott macos env in AzurePipelines times out since some time.

Edit:

https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops

[AArnott]

@amis92 I see you already fixed the macOS problem. :)