Suppose I have a solution with 3 projects:

* Core
* UI
* Tests

Some of the NuGet packages I use will apply to all 3 projects. Some will just apply to UI and Tests, and some will just apply to Tests (like NUnit).

What is the **right** way to set this up using NuGet? 

1. Should I use "Add Library Package Reference" on all three projects any time I need a reference?
2. Should I use "Add Library Package Reference" the first time I need a package, and then use Add Reference->Browse for subsequent usages?

In either case, how many *packages.config* files should I have?