`opam install . --deps-only` do not comply with packages constraints

[rjbou]

Example (hand written)

$ opam install pkg
$ opam show pkg -f depends
"foo" { = "1" }
$ grep depends pkg/opam
depends: "foo" { = "2" }
$ opam install ./pkg --deps-only
The following actions will be performed:
=== recompile 1 package
  - recompile pkg     1 (pinned) [uses foo]
=== upgrade 1 package
  - upgrade   foo 1 to 2     [required by pkg]
[...]

Here, pkg should be removed as its constraint foo=1 is no more satisfied.