bug#39151: Dry-run doesn't say everything

[Julien Lepiller]

On a recently installed system, I ran guix install coq -n. Guix told me it 
would download 270 MB and do 4 hooks. Guix install coq however told me it will 
521 MB (which it did) including a lot more packages (including coq:ide),the 
same 4 hooks with different store hashes and additional grafts. This was quite 
unexpected :)

Once coq is in the repo though, guix gc will remove coq:ide and dependencies, 
and re-installing coq in a new profile will not pull coq:ide in anymore. Note 
that coq was grafted.