pkg: fix dev-tool bug where dune fmt would revert file

[gridbugs]

The rule for running ocamlformat on source files when dev-tools are in use did not depend on the input file, so changes to the input file wouldn't cause ocamlformat to be re-run on the updated file. The consequence of this is that dune fmt would promote stale versions of files.

[gridbugs]

Fixes #10991

[moyodiallo]

Would you like add the test from #10993 ?

[rgrinberg]

Fix looks good, but I think we need to improve things further to avoid mistakes like this. I would suggest the following steps:

1. Demonstrate the bug in a test
2. Make sure the formatting action is sandboxed when using package management
3. Apply this fix

It might be easier to do 2. followed by 1.

EDIT: I see that the test has been created already. Let's take the opportunity to add sandboxing to demonstrate that this dependency is indeed missing.

[gridbugs]

2. Make sure the formatting action is sandboxed when using package management

How would sandboxing the formatting action prevent mistakes like this?

[rgrinberg]

If sandboxing was an enabled for the rule, the action would always fail with some error saying that it was unable to find the input file. Without sandboxing, the rule sometimes succeeds b/c the input was already in _build due to being needed for some other rule.

[gridbugs]

If sandboxing was an enabled for the rule, the action would always fail with some error saying that it was unable to find the input file. Without sandboxing, the rule sometimes succeeds b/c the input was already in _build due to being needed for some other rule.

Ok I added sandboxing and confirmed that without the fix the formatting action can't find its input file.