refactor(pkg): VarSet should just store id's #29407

Job	Run time
Nix (macos-latest)	2m 4s
Build (macos-latest, 5.1.x)	13m 16s
Nix (ubuntu-latest)	51s
Build (ubuntu-latest, 5.1.x)	8m 30s
Coq 8.16.1	7m 4s
Documentation	53s
Format	1m 5s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 31s
Build (4.14.x, ubuntu-latest, true)	1m 27s
Build (4.14.x, macos-latest, true)	2m 16s
Build microbenchmarks	2m 32s
Build (5.1.x, macos-13, true)	3m 50s
Create local opam switch (macos-latest, 5)	5m 35s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 7s
Create local opam switch (macos-latest, 4.14)	5m 52s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 10s
Create local opam switch (ubuntu-latest, 5)	4m 54s
Build monorepo benchmark docker image	3m 41s
Build (4.08.x, ubuntu-latest, true)	1m 37s
Create local opam switch (ubuntu-latest, 4.14)	4m 55s
Build (4.04.x, ubuntu-latest, true, true)	4m 43s
Wasm_of_ocaml	10m 31s
Build (4.02.x, ubuntu-latest, true)	4m 8s
	1h 53m 32s

Provide feedback