dune should give priority to libraries in OPAM_SWITCH_PREFIX instead of ocamlc -where

[kit-ty-kate]

Doing this would prevent https://discuss.ocaml.org/t/i-cant-install-ocaml-lsp-server-and-ocaml-format-lib/15518/6

Currently packages installed by opam are ignored if they are also installed system-wide when using a system-wide installed compiler, i think this is undesirable in the large majority of cases. For people who really want the old behaviour this is changeable via OCAMLPATH anyway.