Define resolve_parent to memoize calls to prefix_substitution that were not (Dot cases)

[gpetiot]

No description provided.

[Julow]

This has a very large impact on the slowest module to compile (Html_sigs in tyxml), which contains a large number of types.

src/xref2/test.md is failing and I don't understand why at the moment (dune build @runmdx). t no longer resolves in this example:

module A : sig
  module M : sig type t end
  module N = M
  module O = N
end

type t = A.O.t

[gpetiot]

I've defined resolve_and_lookup_parent that calls resolve_parent (a glorified call to resolve_module, but the return type is not the same so it looks worth it to me to memoize it separately) and lookup_parent (pre-existing), and apparently dune build @runmdx now runs.

edit: my last commit just re-uses resolve_module instead of defining a new resolve_parent function, if that looks better let's squash the commits before merging.

[Julow]

This shouldn't be a draft anymore!

Can you rebase on top of #1034 to see the result on bigger libraries?

[Julow]

This seems to be a general improvement to the correctness and performances. The double lookup reduces the amount of code and the number of caches, which seems to be the right direction.
I'd rather merge this before more work can be done on this part of the code.

[Julow]

There have been concerns about the double lookup but this PR is already an improvement. More improvements should be made on top and should not block this.
Let's merge before it rots.