Option to dump metadata in a machine-readable format

**Feature request:**
Add a new command to dump metadata as JSON or any other machine-readable format.

Output could be used by tools like [upstream-ontologist](https://github.com/jelmer/upstream-ontologist). There are 3rd-party `.cabal` parsers but they're all wrong - the format is hard to parse.

The issue names no source files, tests, or entry points. Start by reviewing upstream-ontologist and the limitations of third-party .cabal parsers; done means a new command can dump metadata in a stable machine-readable format such as JSON.

Written by the indexing model from the issue text.