is duplicated by:

• SR-13265 SwiftPM ModuleMapGenerator should use DiagnosticsEngine

Issue Description:

SwiftPM's ModuleMapGenerator currently emits any warnings to `stdout` and throws specialized errors in case of problems. It should use DiagnosticsEngine like the rest of SwiftPM.

The output to `stdout` is particularly problematic since it can interfere with things like `swift package describe --type json` (which, by the way, is somewhat fragile already, since any `stdout` output can interfere with it — better to provide a way to emit the JSON to a pipe or file, but that's a separate JIRA).