Support atomic environment updates in OpamFile

[dra27]

Extracted from #5417, which now just deals with the fully revertible environment updates part.

• Writing the environment file in opam env is still taking the switch write lock, which isn't necessary
  • we keep taking write lock as something is written in switch
• Add hook specific read-only option: it is like read-only, but permit environment files writing.
  • We want to keep hooks as readonly. The only downside is that when a local switch is moved, the hook takes some times (switch loading) to recompute environment ; prompt will take that time until an opam commands in the terminal with write lock writes the new environment
    • We can still add the hook-read-only (read-only except for env) in the future, when it makes sense to move a switch
• Test for moving a switch (verify PATH updates)

[rjbou]

lgtm!
I updated api changes.

[dra27]

Test added - up to you whether we want to deal with the lock before or after merge, @rjbou?

[rjbou]

From dev meeting: add hook specific option to permit read-only except for environment file ; it resolves switch lock issue in the same time as there is no clash possible on environment files name (temporary files)

[kit-ty-kate]

Is there a downside to always having all file writes as atomic?

[kit-ty-kate]

When i tried to implemented this feature before i ended up to the same conclusion but only because i didn't think of using Filename.open_temp_file ~temp_dir:dst_dir which takes care of the only problem i was thinking of, being: x.tmp already exists.

[rjbou]

Atomic takes few more steps: first write on tmp, then rename on final location, instead of directly writing on final location.
related #5473

[kit-ty-kate]

Yes but that's fine. I don't think i'm able to see any usecase where this isn't better

[kit-ty-kate]

Thanks