Meta issue on the problems encountered wrt the new compiler variants layout

[kit-ty-kate]

See several issues encountered by users: https://discuss.ocaml.org/t/experimental-new-layout-for-the-ocaml-variants-packages-in-opam-repository/6779

• One of the issue is the lack of ways to install a compiler variant directly using opam init (related to Add support to initialise opam with multiple repositories #4555)
• (2.1 regression) opam switch create . 4.12.0+options --packages=ocaml-option-flambda worked in opam 2.0 but not in 2.1 (CLI regression tracked in CLI regression in opam init in 2.1.0 #4843)

(list non-exhaustive, I'm mainly opening this issue to get things written somewhere)

[dra27]

The first obviously has an easy workaround (opam init --bare followed by opam switch create) but as in #4555, it is obviously better if opam init is essentially able to do everything opam switch create can, so in addition to --repositories, opam init ought to support --packages and --formula and --compiler ought to be marked deprecated.