[bug#34958] Add a package argument to dune-build-system

[Julien Lepiller]

Hi, these two patches add a new #:package argument to the
dune-build-system. This is required to build a package that comes with
more than one build file, such as packages in a repo with multiple
packages.

This is the case for ocaml-alcotest that is updated in the second
patch. It used the ocaml-build-system, but the build system was changed
upstream to dune, so the first patch is needed to build alcotest. I'd
like some feedback especially on the first patch :)

Thank you!

0001-guix-dune-build-system-Add-a-package-parameter.patch
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0002-gnu-ocaml-alcotest-Update-to-0.8.5.patch
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