Dune should not look up other sub-directories when given `-p`

When investigating the issue ligo's developers have here https://github.com/ocaml/dune/issues/2131#issuecomment-550475432 with @rgrinberg, we discovered that, when given a directory containing two sub-directories with two projects like so:

+ /
  - dune-project
  + project-a/
    - dune-project
    - project-a.opam
    - [...]
  + project-b/
    - dune-project
    - project-b.opam
    - [...]

dune build @install, even when given -p project-a, will scan for every packages used by project-b, thus, if opam install some of them at the same time, a race condition can occur in the same way as it happened in https://github.com/ocaml/dune/issues/2131

Here is a sample of the strace for dune build -p ocplib-resto with this archive:

stat("/home/kit_ty_kate/.opam/4.07.1/lib/ocaml/stdlib.dune", 0x7ffed0b53020) = -1 ENOENT (No such file or directory)
stat("/home/kit_ty_kate/.opam/4.07.1/lib/tezos-protocol-environment-sigs/dune-package", 0x7ffed0b530c0) = -1 ENOENT (No such file or directory)
stat("/home/kit_ty_kate/.opam/4.07.1/lib/tezos-protocol-environment-sigs/META", 0x7ffed0b530a0) = -1 ENOENT (No such file or directory)
stat("/home/kit_ty_kate/.opam/4.07.1/lib/META.tezos-protocol-environment-sigs", 0x7ffed0b530a0) = -1 ENOENT (No such file or directory)

Start by reproducing the dune build @install and dune build -p project-a commands with two nested projects, then trace how Dune discovers projects and packages. Done means selecting project-a no longer scans or looks up packages from project-b, while the reported build still works.