`opam switch create some-name ocaml-option-tsan` installs BER MetaOCaml

[OlivierNicole]

Title. To quote @rjbou:

the reason: ocaml-option-tsan depends on ocaml-variants and the solver chooses ocaml-variants.x.y+BER which have meta-ocaml as post dependency (instead of ocaml-variants.x.y+options)

and @kit-ty-kate:

the metaocaml packages should probably have flags: avoid-version to avoid this. If no version is requested (either in the command line, or in the package's dependencies) opam will choose whatever is the latest version is (modulo avoid-version). In this case though it chooses the version that has less overall dependencies i think, which might be a bad solver decision