Improve ability to programmatically consume bst output

There's some things inside bst that would be useful outside bst as well, eg sources and artifact file list in generation of manifest. bst artifact list-contents is a thing, however bst source list-sources is not. And it's not possible to make the output programmatically consumable (eg JSON) through a parameter. Let's please improve upon this story before bst2 to provide official interfaces rather than needing to hack at API's which may or may not be public to provide this information.

Start by comparing the existing `bst artifact list-contents` command with the requested `bst source list-sources` interface and the manifest-generation output. Investigate how command output is currently produced and whether a parameter can provide JSON or another machine-readable format. Done means officially supported, programmatically consumable interfaces cover the requested source and artifact information.

Written by the indexing model from the issue text.