Importing just exported switch workflow broken

[rjbou]

From #2211

@dbuenzli wrote:

Now that compilers became packages this no longer works:

 * No agreement on the version of ocaml-base-compiler:
  - (invariant) → ocaml-base-compiler >= 4.14.0
   - ocaml = 4.12.0 → ocaml-base-compiler < 4.12.1~
   You can temporarily relax the switch invariant with `--update-invariant'
No solution found, exiting

I certainly do not want to relax the switch invariant since this will result in having in 4.12 in my 4.14 switch.

Underlying issue is that compiler packages are not well stored with switch invariant setup

related to #4572