improvement: Resolve console REPL artifacts via coursier API instead of the CLI

[jozanek]

Stacked on fix/1226-ammonite-console (upstream PR scalacenter#2941); review shows just the single follow-up commit.

bloop console shelled out to a coursier/cs launcher on the client's PATH and requested ammonite_<exactScalaVersion>:latest.release. So it failed with a bare "command not found" when no launcher was present, and with a raw coursier stack trace when Ammonite wasn't published for the project's exact Scala version.

This resolves the REPL artifacts with bloop's existing coursier API (DependencyResolution) and emits a plain java -cp ... <main> command instead:

• no dependency on an external coursier launcher (only java is needed);
• resolution failures surface as a clear, actionable message;
• also fixes a latent bug where the Scala-version fallback for Java-only projects produced a garbage artifact coordinate.

Applies to both the Ammonite and Scala REPLs. ConsoleSpec now launches the generated command end-to-end and asserts the project's classes are visible in the REPL.