SBT plugin does not resolve .courier references from dependencies

[kchen-coursera]

The plugin currently only resolves files in the <lib>/pegasus directory but does not offer a simple way to configure or resolve .courier files from a dependent library. This leads to minor annoyances with shared models from other packages that you want to reference in courier sources and is a blocker for splitting up a centralized models repository. Note that courier codegen classes are resolvable because that is post-SBT plugin resolver code path.