Support composite selection from an external collection of repositories, removing the need to have more than one copy of each repo just for selecting different composite builds

[calvertdw]

I think this entails a selector configuration file (that is gitignored?) that selects the build roots to include.

It still requires the CompositeBuildAssembler in order to not need to list every single subproject that you need.

[calvertdw]

@dljsjr I made this issue for later reference.