Change the default repo layout to match dotnet/Arcade

[natemcmaster]

Part of incremental changes to move to arcade. dotnet/arcade#88

Arcade repo layout prescribes putting bin/ and obj/ folders in $repoRoot/artifacts/, and putting infrastructure stuff in eng/. We can update aspnet repos to use this layout now as a stepping stone to move to arcade.

See https://github.com/dotnet/arcade/blob/master/Documentation/ArcadeSdk.md

- eng/
	+ *.props/targets
- artifacts/
	+ tmp/
	+ log/
	+ packages/
         - bin/
             + $(ProjectName)
	- obj/
              + $(ProjectName)