Use a Dune dialect to preprocess eml templates

[aantron]

This spiritually resurrects #1181 by @gpetiot which stalled, as I understand it, due to lack of a Dream release that would make dialect support ergonomic. Because of the Dream release, the present PR doesn't need a wrapper around dream_eml.

It should also make #2764 unnecessary.

[cuihtlauac]

Thanks @aantron

In #1181, @emillon made this comment:

Sidenote, I had a look at the diff in this PR. If I understand correctly you're replacing N rules with each 1 dependency and 1 target, by a single rule that has N dependencies and N targets.

What's the status now? Are all the .eml going to be processed if a single one changes?

[aantron]

AFAIK that comment referred to an approach that was being used in that PR (12c9134#diff-85098e336ea5f1f8dca91cda539ff5f46db068c37b959eb70a32d02bc591941fL6) that had all the template outputs depend on all the template inputs, before @gpetiot switched to using a Dune dialect in the commit from that PR that I am linking in this comment. I also had that concern for a moment before I realized that, but AFAIK there is no reason to suspect that Dune dialects have this downside.

[sabine]

LGTM thanks @aantron!