expunge: move system code to dune

[emillon]

The logic to create utop from utop-full is as follows:

• use ocamlobjinfo on utop-full to determine which modules are used
• use ocamlobjinfo on ocamlcommon, ocamlbytecomp, and ocamltoplevel to determine which modules to exclude (a priori)
• compute the set used - (exclude - {Topmain,Toploop,Topdirs})
• call expunge with this set of modules

Previously, this was all done in expunge/expunge.ml, meaning core logic

• process handling. What this PR does is offload the process handling to dune: calling ocamlobjinfo and expunge is done instead in dune rules, and expunge/modules.ml is just about reading sets of module names and outputing the result.

Besides simplicity and portability, the other advantage is that the intermediate modules.txt can be inspected directly.

[emillon]

this works with sandboxing, and I confirmed (after fixing a typo) that it produces the same binary,