more flexible libcurl depext on debian/ubuntu

[madroach]

libcurl4-openssl-dev works as well as the gnutls variant. Therefore depend only on the libcurl-dev virtual package, which makes sure that any variant of libcurl4-*-dev is installed.

[madroach]

Hmm, the tests show this does not always work without help of the user.
Opam will now detect any installed libcurl variant.
But when libcurl is not yet installed apt-get install libcurl-dev will fail and ask the user to provide a specific package that will provide libcurl-dev.
This might be desirable behaviour. I don't know.

[avsm]

It's probably better from a UI perspective to have two conf packages (which depend on the variants) and let the opam solver just pick one. A user can select a concrete one to override if they have a preference, then.

[madroach]

Sounds good, but how do I depend on alternatives?

[avsm]

Just a a | b in the depends string works

[avsm]

just one last request is to rename conf-libcurl.2 to a new conf-libcurl.3 (with the current content), so that existing locked packages wont be affected.

[avsm]

This is causing a lot of temp failures, i've restarted:

+ /usr/bin/sudo "apt-get" "install" "-qq" "-yy" "libcurl4-openssl-dev"
- E: Failed to fetch http://deb.debian.org/debian/pool/main/c/curl/libcurl4-openssl-dev_8.14.1-2%2bdeb13u4_amd64.deb  Temporary failure resolving 'deb.debian.org'
- E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
[ERROR] System package install failed with exit code 100 at command:
            sudo apt-get install -qq -yy libcurl4-openssl-dev
[ERROR] These packages are still missing: libcurl4-openssl-dev

[NOTE] You can retry with '--assume-depexts' to skip this check, or run 'opam option depext=false' to permanently disable handling of system packages.

[madroach]

Those network failures were caused by attempts to fetch deb packages in a sandbox without network. The problem was that opam tried to switch between the openssl and gnutls variants without realizing that the underlying debian packages conflict. Noting this conflict in the opam conf- packages fixes most of the failures.

The only remaining additional CI failure in comparison to the state of the repository before this PR (af2e0b5) is a failure in testing opentelemetry-client-ocurl.0.6. This is caused by an attempt to upgrade ocurl from version 0.9.2 to transition (deprecated). All other versions of opentelemetry-client-ocurl build and test just fine. This seems to happen because opentelemetry-client-ocurl.0.6 is the only version which depends on opentelemetry-client-cohttp-lwt for testing.

However that may be — all those failures are caused by attempts to download sources in a sandbox and not by a problem with this PR. Therefore to me it seems this is ready to commit now.