Split the manifest into smaller components.

The following discussion from !5157 (merged) should be addressed:

• @raphael-proust started a discussion: (+2 comments)

  I feel like the manifest is starting to get quite big. Could we split it in smaller chunks of code? That's for easing reviews and maintenance. I'm thinking:

  • one file for all the helper functions
  • one file for declaring packages (or possibly multiple files: one for everything under src/, one for devtools/ etc. but that's something we can discuss pros-and-cons)
  • one file for each of the generation target (so like one file for generating dune and opam files, one for the CI)

  Ideas? Opinions? Counter proposals?