pkg_in_pipe: write a machine readable version of the report

[glehmann]

The report is written as a json file that other scripts can consume,
e.g. to generate the release post or to move the plane cards of the
builds to another state.

Signed-off-by: Gaëtan Lehmann gaetan.lehmann@vates.tech

This PR is part of a tree containing 4 PRs:

1. master
2. pkg_in_pipe: keep PRs merged by rebase or squash #838 → master
3. "pkg_in_pipe: write a machine readable version of the report" (this PR) → pkg_in_pipe: keep PRs merged by rebase or squash #838
4. pkg_in_pipe: move the plane cards of the builds of a tag to a given state #840 → pkg_in_pipe: write a machine readable version of the report #839
5. pkg_in_pipe: generate a draft of the release post from the report #841 → pkg_in_pipe: write a machine readable version of the report #839