Handle dependency flow across different releases

The .NET Core SDK has some dependencies that come from previous release bands. This includes the latest patch versions for previous versions of .NET Core, as well as templates targeting previous versions.

Right now these dependencies are managed manually. We should figure out how to automate the dependency flow.

A possible solution would be to include the Major.Minor version of .NET Core in the package IDs that need to flow across release bands. This is already the case for the template packages. We could create a new package to replace Microsoft.NETCore.App which includes the latest patch version, the list of RuntimeIdentifiers for which there are runtime packs, and other information that we currently get from Microsoft.NETCore.App.

We would probably also need to add the ability to filter the packages which update in a dependency flow edge. Otherwise a build of 3.0 (for example) would result in those packages flowing to 3.1 and 5.0.

@mmitche @livarcocc @dagood @ericstj