compile-p-option-broken

This is why `cs3110 compile -p core file.ml` didn't work, but running `echo "compile.opam_packages = core" > .cs3110; cs3110 compile file.ml` did. Not sure if this was an oversight this summer or a deliberate choice: "if you're passing them explicitly we'll do exactly what you want". 

If it was deliberate, I think it was a bad deliberation.

@andymw @arjunbiddanda @clarksmr 


This matches the behavior in line 223 of `util/cli_config.ml`.