Establish a pattern/helper for handling solutions and traversal projects in a similar way

### Is your feature request related to a problem? Please describe.

If folks are going to have success using Traversal projects with `dotnet`, we need command implementations to be able to detect and work against solution files and Traversal projects in a unified way. In general, commands should be asking "is this a collection of projects or a single project", and we should have answers for how to get the collection of projects from whatever input is provided - solution or Traversal project.

Once the set of projects to work on is computed, command implementations should be able to 'process' each of those discovered projects. In this way we separate the steps logically and open the door to future changes to the set of things we consider 'buildable' or 'interactable'.