fix(watch): track looked up binaries

[rgrinberg]

Now dune's watch mode will be responsive when binaries it uses changes.
Note: some basic binaries are exempt from this ($vcs, cygpath, bash, sh,
etc.)

[ghost]

Looks good once add_exe gets a doc comment.

[ghost]

This seems to have broken things. Now I'm getting:

Error: inotify_add_watch: /usr/local/bin: No such file or directory
-> required by %{bin-available:hg} at test/blackbox-tests/test-cases/dune:153

[rgrinberg]

That's weird, the directory clearly exists for you. Any idea what's happening @snowleopard

[ghost]

It doesn't actually. But it is in my PATH. We have to assume that some of the directories in the PATH might not exist and cope with it.

[rgrinberg]

I see. So you don't even have /usr/local?

We should definitely handle this. Will send a fix

[ghost]

I have /usr/local but not /usr/local/bin

[ghost]

@rgrinberg did you get a chance to look at this? I still have edit my PATH every time I start working on Dune

[rgrinberg]

I fixed it for the fsevents backend in #4990. Can we get that PR merged and I'll do a fix for inotify. I've crammed enough into one PR already :)