[refactor] clean up 0install codebase further

[ElectreAAS]

This PR is a follow-up to #11015, in that it cleans up the now un-vendored opam-0install/0install directories. The two commits are independent and can be reviewed separately.

[rgrinberg]

Ci doesn't seem happy on 4.14:

cd _boot && /home/runner/work/dune/dune/_opam/bin/ocamlopt.opt -c -g -no-alias-deps -w -49-6 -alert -unstable -I +threads zeroinstall_solver__Diagnostics.ml
File "src/0install-solver/diagnostics.ml", line 416, characters 22-37:
Error: Unbound value RoleMap.of_list
make: *** [Makefile:32: _boot/dune.exe] Error 2

[ElectreAAS]

Thanks CI, for exposing that I'm an idiot 😅 The failures on MacOS seem weird though, something related to patch?

[Leonidas-from-XIV]

Yes, we'll need to fix this ASAP. It is not related to this PR so for what it's worth this PR passes CI.