Add missing java packaging

[vincent-botbol]

This PR fixes the missing installation of the Java files. We (@AltGr & myself) figured that copying over the Java files as is would be better than to depends on Java. Hence, it's the user's responsibility to compile the java files as needed.

[rmonat]

Looks good to me, thank you!

Could you maybe add a runtest target executing the java tests as well? No need for the dependencies to be added in the opam file, but I think it would be nice to still have the tests running for all implementations of dates-calc.

[vincent-botbol]

Looks good to me, thank you!

Could you maybe add a runtest target executing the java tests as well? No need for the dependencies to be added in the opam file, but I think it would be nice to still have the tests running for all implementations of dates-calc.

Following offline discussions, ocaml/opam-repository#27813 has been merged. We can now add the dependency to the opam file and run the java test through dune runtest (given that the CI's opam repository is up-to-date).