Make yaml work in monorepos

[TheLortex]

This PR is in two parts:

• first an upgrade to dune 2.0
• then two changes to make yaml work in monorepos:
  • use realpath to make ctypes' include path absolute, because %{lib:ctypes:ctypes_cstubs_internals.h} is relative in a monorepo. I have added a C stub for it, because Unix.realpath only exists since 4.13.
  • add explicit dependencies for all the header files that are used. Otherwise they don't get installed.

[TheLortex]

@avsm A 3.0.2 release with this PR would be very useful for mirage-www and the irmin monorepo work !

[avsm]

This probably breaks Windows due to the C stub. I'm ok with a new release of Yaml that's 4.13+ only and using Unix.realpath, as that's going to be our practical minimum version for Mirage anyway.

Also instead of the absolute path, shouldn't it be normalised to the project root? Otherwise reproducibility is affected.

[TheLortex]

This probably breaks Windows due to the C stub. I'm ok with a new release of Yaml that's 4.13+ only and using Unix.realpath, as that's going to be our practical minimum version for Mirage anyway.

Okay, I'll update the PR.

Also instead of the absolute path, shouldn't it be normalised to the project root? Otherwise reproducibility is affected.

I do'nt think reproducibility is affected.
Indeed the intermediate file ctypes-cflags won't be reproducible, but it is not installed, only used in the following command: run %{cc} %{c} -I../../vendor %{read-lines:../../config/ctypes-cflags} -I %{ocaml_where} -o %{targets})) whose result is independent of the project root.

[avsm]

Sounds good. It does affect relocatability, but we can deal with that once OCaml itself supports it.

[samoht]

Would be nice to merge and release this as we need it to build mirage-www.