Support a collection of packages

[denelon]

As a developer I want to be able to specify a set of packages for the Package Manager to install.

Ideally this would allow a single file to be able to drive a more complex scenario that supports installing multiple packages. The shapshot ask would likely output it's results in this form. The collection could reference multiple manifests in one or more repositories. Initially the scope might need to be reduced to a single repository. If that's the case, this Issue should be split.

[masx200]

I support it!

[denelon]

This is not the same thing as #219

[mimi89999]

Is this what other package managers call virtual packages? A package that does not have files or an installer in itself, but only has dependencies.