[bug] provides doesn't allow for package conflicts

Environment Details (include every applicable attribute)

• Operating System+version: Linuxmint 20 ulyana
• Compiler+version: gcc 9
• Conan version: 1.30.2
• Python version: 3.8.5

Steps to reproduce (Include if Applicable)

Set conanfile.txt to include dependencies for qt/5.14.1@bincrafters/stable, opencv/3.4.5@conan/stable and libjpeg-turbo/2.0.5. libjpeg-turbo says that it provides libjpeg however the version mismatch between qt and opencv requires you put libjpeg in your conanfile.txt. This check should also check if another library provides libjpeg and treat that as a resolution between the conflict.

Logs (Executed commands with output) (Include/Attach if Applicable)

ERROR: Conflict in qt/5.14.1@bincrafters/stable:
    'qt/5.14.1@bincrafters/stable' requires 'libjpeg/9d' while 'opencv/3.4.5@conan/stable' requires 'libjpeg/9c@bincrafters/stable'.
    To fix this conflict you need to override the package 'libjpeg' in your root package.

[SpaceIm]

https://github.com/conan-io/conan-center-index/pull/2948 aims to provide a virtual libjpeg recipe, so that all others recipes depending on libjpeg could require this virtual package -avoiding to reimplement again and again this logic in each recipe- and solve this issue (it would ensure a clean dependency graph without mutually exclusive libjpeg implementations). Then consumers would just have to select libjpeg implementation (libjpeg, libjpeg-turbo or mozjpeg) through libjpeg virtual package option.

Unfortunately, there is currently no progress in this PR. I can reopen a PR for this recipe (from existing branch), since I also have interest in such virtual package.

[Croydon]

That is, you might want to use https://conan.io/center/opencv package, which will be opencv/4.3.0, without the conan/stable part.

FYI, openvc/4.3.0 is not yet in CCI. This is a bug in the online UI which is not displaying username/channel anymore, see also https://github.com/conan-io/conan-center-index/issues/3360