Use coursier for SBT project itself

[andyscott]

If we are using Coursier, we should go all out by adding the plugin in project/project/plugins.sbt for our projects. Ideally the policies plugin will keep the version of coursier in sync with the version used by sbt-org-policies itself.

Adding coursier to the nested project folder will force most of the project dependencies to be downloaded via coursier instead of ivy. This excludes the SBT bootstrapping dependencies, which will still be resolved via traditional Ivy resolvers.