opam install --deps-only <path> should not upgrade packages from that path

[kit-ty-kate]

Weird behaviours can happen if a package being requested to install only its dependencies is already installed itself:

$ cd opam
$ opam install --deps-only .
[...]
=== recompile 67 packages
[...]
  ↻ ocaml-base-compiler  4.08.1                         [upstream or system changes]
[...]
=== upgrade 6 packages
  ↗ opam-client          2.2.0~beta1 to 2.3.0~alpha~dev [upstream or system changes]
  ↗ opam-core            2.2.0~beta1 to 2.3.0~alpha~dev [upstream or system changes]
  ↗ opam-format          2.2.0~beta1 to 2.3.0~alpha~dev [upstream or system changes]
  ↗ opam-repository      2.2.0~beta1 to 2.3.0~alpha~dev [upstream or system changes]
  ↗ opam-solver          2.2.0~beta1 to 2.3.0~alpha~dev [uses opam-0install-cudf]
  ↗ opam-state           2.2.0~beta1 to 2.3.0~alpha~dev [upstream or system changes]

The resulting switch contains packages that are neither from a repository nor from an kind of pin.
opam pin and opam list after that command show:

$ opam pin
[blank]

$ opam list 'opam-*'
[...]
opam-core            2.3.0~alpha~dev Core library for opam 2.2
[...]