Fix the opam-devel package + various fixes

[kit-ty-kate]

Following the merge of #4178, the opam-devel package now doesn't install anything anymore (see the special opam-devel.install makefile rule). 1471a27 should fix that.

Otherwise the rest is more of a collection of various fixes that can be taken independently:

• 8d4ae0d is here because the opam-%.install rule is still used in the makefile (see the installlib-% rule for instance)
• 5429c09 fixes an issue that happens when an external archive is not available to download and the script tries to get it from the cache but the url does not exist anymore.
• 9f3dacf is a preventive fix in case someone changes the name of the opam binary (in the src/client/dune) it will not try to use the opam binary from the PATH (which, if it is not opam 2.1 already is going to hang up forever trying to get opam lock --help=groff
• 866fbc7 fixes an issue in the opam file as dune is not compatible with the {build} tag anymore (see [RFC] relax parsing of dune-package files dune#2147)

[rjbou]

Thanks! A rebase, update changelog and it'll be perfect.

[AltGr]

Rebased

[dra27]

This change breaks the mingw builds - what's it fixing elsewhere?

[kit-ty-kate]

oh.. I thought dune would have dealt with that and replaced the path

[dra27]

Ah, looking at the description of the PR I see the answer to my question. doc/man/dune must use %{bin:opam} - the fix instead might be to a put a comment above public_name opam in src/client/dune noting that if it's changed that it needs to be changed elsewhere. To be honest, I'm not totally sure why we should be hardening the build system against external changes 🙂

[kit-ty-kate]

The problem I was trying to solve arise when I was trying to remove the dummy opam.opam package/file and fix the opam-devel package without reintroducing a custom Makefile rule.
Since normal dune packages install their opam file in %{lib}%/opam-devel/opam (the same place opam is installed right now), I had to change the name of the binary. It seemed to worked locally (since I had opam 2.1 already installed) but I ran into an infinite loop when I tried to compile it in an environment that had opam 2.0.

But I agree this change was optional and since it fails with mingw I removed it from the PR

[dra27]

Hopefully the change in #4264 should make that error less weird if you hit it again in future, @kit-ty-kate!

[AltGr]

Since normal dune packages install their opam file in %{lib}%/opam-devel/opam (the same place opam is installed right now)

😱 ah I get it now...
100% dune's fault though 😇

[rjbou]

Thanks!