feature: empty alias

[Alizter]

This grants #4161.

We add an @empty alias allowing the user to build nothing. We also add a check so that user defined rules are not added to the empty alias.

I don't know if we want to or need to be more strict on the checks. For instance, we can bypass the simple rules and add things to the empty alias, but it seems pointless to check for this all the time.

• changelog

[Alizter]

TODO: make this error message more in line with other similar errors

[Alizter]

OK so there is a weird constraint here. I've restricted user-defined rules from adding things to the empty alias however a cram test, like we have in the repo, called empty.t will then fail when it generates the empty alias.

This is why the CI is failing because we are failing on this restriction after bootstrapping.

@rgrinberg @snowleopard the idea that users should be restricted from adding rules in the empty alias seems like a good one, but how can we ensure it doesn't restrict people creating cram tests with those names?

[rgrinberg]

Maybe you can change the test specific aliases to just be the full filename of the test starting from the next version of dune? You can still keep the old names as well, but just warn about empty.t

[rgrinberg]

Merging this and the restriction on the @empty alias can be introduced starting from the next dune version. We can improve things further over time.

[maiste]

This is mostly to not forget about it but shouldn't an entry be added in the reference manual for the new alias?