Complex Packaging ready for use

[markwilkie]

For the most part, teams should simply use 'dotnet pack' to package. However, there are certain situations which require a more complete solution - namely corefx and asp for now.

This epic will be considered complete when:

• ASP and CoreFx are agreed on the requirements
• The new package which fulfills these requirements is in use by at least one repo from CoreFx and ASP.

[markwilkie]

In talking w/ @weshaggard, this effort will likely come out of moving functionality out of buildtools into Arcade.

cc/ @natemcmaster

[weshaggard]

@ericstj has done this work for CoreFX with #1030.

[markwilkie]

thanks guys!