Indentation of opam code

[rjbou]

The current/usual indentation of opam done is done using ocp-indent. We need to enforce and share some indentation rules like that there is no indentation only diffs in proposed PRs.
We also need to re-indent the whole code for once.
Discussed in dev meeting, we should do that once opam 2.2.0 is tagged, like that there is no issues with current opam 2.2 PRs.
There is also discussions to have on how to enforce using or not CI jobs.