support non-maven resolvers

[softprops]

the ivy flavor of sbt resolvers is not currently supported. Users to use the externalResolver to override the sbt default (scala tools) are currently greeted with this unfriendly message

ls currently only supports sbt MavenRespositories which URLRepository(sbt-plugin-releases,sbt.Patterns@77252b15) is not.

sbt plugins move to the new ivy repo I should also support URLRepositories

The relevant code to adapt is here

Note that the client will have to somehow detect this when resolving libraries from ls which means I may have to encode a resolver as more than just a simple string