Feature request: Include sourcemaps in built jspm packages (external .map files)

When I debug my application, it would be very useful to be able to step into the Aurelia code with the source maps. It's also a great way to learn the framework.

Start by tracing how the framework's built jspm packages are produced and where their package contents are assembled. Verify how source maps are currently generated or omitted, then confirm that the finished packages include external .map files that let applications step into Aurelia code while debugging.

Written by the indexing model from the issue text.