Support --json option in opam tree

[cannorin]

Closes #5298.

This PR adds support for --json option in opam tree command.

Here is an example output: https://gist.github.com/cannorin/ae87f55695c38356a9b880e0418a7a12

Currently, the output JSON has the type Forest in the following TS definition:

interface Package {
  name: string;
  version: string;
}

interface DepsTree extends Package {
  dependencies: DepsNode[];
}

interface DepsNode extends DepsTree {
  satisfies: string | null;
  is_dup: boolean;
}

interface RevdepsTree extends Package {
  requirements: RevdepsNode[];
}

interface RevdepsNode extends RevdepsTree {
  demands: string | null;
  is_dup: boolean;
}

type Forest = DepsTree[] | RevdepsTree[]

We discuss the structure of the output JSON in #5298.

TODO:

• Add tests
• Update doc and man not needed
• Update master_changes.md

[rjbou]

Thanks for the PR!
I've moved the test to json testfile, and added some commands to test (with error, with simulated)

[rjbou]

thoughts from dev meeting: we should add switch name into the json, no need to add error / simulated state

[cannorin]

Oops I broke some other tests in json.unix.test and I'm fixing it now.

[cannorin]

@rjbou All the other subcommands are using the default man for --json, so we don't have to provide man for opam tree --json, right? If so, I think it's safe to mark this PR as ready for review.

[kit-ty-kate]

@smorimoto Could you confirm on whether or not this is what you wanted?

[smorimoto]

@kit-ty-kate Yes! That looks OK! Let's go!

[smorimoto]

When will this be deployed?

[rjbou]

LGTM!
Wondering on some wordings:

• is_dup -> is_duplicate ?
• demands or satisfies are they enough indicative ?

[cannorin]

is_duplicate may be better.

I didn't come up with better terms than demands/satisfies...

[dra27]

is_duplicate and satisfies seem good - might be clearer with the example to figure whether something other demands/requirements wanted (and this is all just fixing names - the fundamental layout looks fine)

[smorimoto]

Updates?

[rjbou]

queued on #5405

[rjbou]

Thanks!

[smorimoto]

Thank you for everyone!