excludes should include local opam (2.0.0) switches

[dysinger]

... otherwise dune build -w produces infinite

 inotifywait (internal) (exit 1) 
/usr/bin/inotifywait -r . --exclude \.#|_build|\.hg|\.git|~$|/#[^#]*#$|\.install$ -e close_write -e delete -q > /dev/null
Failed to watch .; upper limit on inotify watches reached!
Please increase the amount of inotify watches allowed per user via `/proc/sys/fs/inotify/max_user_watches'.
Had errors.
Had errors.                      
 inotifywait (internal) (exit 1) 
/usr/bin/inotifywait -r . --exclude \.#|_build|\.hg|\.git|~$|/#[^#]*#$|\.install$ -e close_write -e delete -q > /dev/null
Failed to watch .; upper limit on inotify watches reached!
Please increase the amount of inotify watches allowed per user via `/proc/sys/fs/inotify/max_user_watches'.
Had errors.
Had errors.                      
 inotifywait (internal) (exit 1) 
/usr/bin/inotifywait -r . --exclude \.#|_build|\.hg|\.git|~$|/#[^#]*#$|\.install$ -e close_write -e delete -q > /dev/null
Failed to watch .; upper limit on inotify watches reached!
Please increase the amount of inotify watches allowed per user via `/proc/sys/fs/inotify/max_user_watches'.
Had errors.
Had errors.                      
 inotifywait (internal) (exit 1) 
/usr/bin/inotifywait -r . --exclude \.#|_build|\.hg|\.git|~$|/#[^#]*#$|\.install$ -e close_write -e delete -q > /dev/null
Failed to watch .; upper limit on inotify watches reached!
Please increase the amount of inotify watches allowed per user via `/proc/sys/fs/inotify/max_user_watches'.
Had errors.
Had errors.                      
Waiting for filesystem changes...^C

[rgrinberg]

Excluding _opam isn't ideal though isn't it. It seems like it's nice to have automatic rebuilds whenever you make a change to the local switch.

I wonder if we can do a more coarse grained watch on the _opam dir.

[ghost]

In fact, because dune completely ignores all directories starting with an underscore or a dot, we shouldn't be watching _opam. However I don't know how to tell that to inotifywait or fswatch.

@rgrinberg, what we would really want is to watch all external dependencies, whether they are in a local opam switch or not. Though that's even more complicated.

For this particular case, given that the current behaviour prevents users of opam local switches from using watch mode, we should just merge this PR.

[rgrinberg]

Alright, will merge manually with an appropriate change entry. Yeah, I guess we shouldn't be discovering external deps by peeking into the switch dir anyway. After all, we get most of the deps from findlib.

[rgrinberg]

@dysinger thank you for your contribution. I've merged it manually to master with an appropriate CHANGES entry.