Resolve declared dependencies over runtime modules

[DanielThomas]

Summary

• retain dependency versions declared by Maven metadata, including optional dependencies
• resolve a requested external dependency even when an older module is present in the runtime image
• continue using runtime modules when no source or Maven dependency declares an external version

Validation

• ja test --all (549 tests)
• assembled jig and resolved ja@0.21.0 from a runtime containing stale tool modules; selected jdocserver@0.4.3 and jfmt@0.7.4 on the upgrade module path