ATLASG-2948: use libcurl instead of ROOT::Cp in PathResolver

In path resolver using https, ROOT currently fails to download generic files. I had a workaround in !82274 (merged); this improves it by:

• Using libcurl directly instead of std::system("curl ..."). The latter is less code, but I'd say invoking another process via std::system is uglier.
• Dropping TFile::Cp: better to have one reliable method than a flaky one with a fallback.

I'm a bit worried about one point: I was unable to build this package from a container, because ATLAS CMake failed to find libcurl in a container. But it works fine on a machine with cvmfs.