melange: allow to define copying assets rules from the libraries

[jchavarri]

A potentially common scenario is having multiple melange.emit stanzas in the same dune project, where these emit stanzas consume the same libraries.

Currently, the dependencies on assets inside the library have to be defined using the emit alias, e.g. if a library foo depends on some asset bar.svg, the dune config will look something like:

(melange.emit
 (target output)
 (alias mel)
 (module_system commonjs))

(copy_files
 (alias mel)
 (files bar.svg))

The issue with this approach is that in a scenario like mentioned above, where there can be multiple usages of a given library by multiple emit stanzas, so users will need to write the copy_files rules for the library file N times unnecessarily.

One solution could be to define the dependency on the assets at the library level, e.g.:

(library
 (modes
  (mode
   (name melange)
   (assets bar.svg))))

[jchavarri]

I am not a big fan of the proposed solution with library.mode.assets. If this use case is generalizable to any library mode —not only melange, but also byte and native—, then it might be preferable to just add a new field runtime_deps inside the library stanza?

[rgrinberg]

I would suggest a melange.runtime_deps field. I'm not sure how this field would be used outside of melange so a general runtime_deps field is premature. Sticking it into modes also isn't the style of how dune does things.

[jchavarri]

it turns out, I had already started looking into implementing this #6643. I just updated the PR and asked a question to understand how to proceed next in https://github.com/ocaml/dune/pull/6643/files#r1098380524.

[anmonteiro]

This was done in #7234