Only consider opened projects when computing a project's direct dependencies

When computing a project's direct dependencies, filter out non opened
projects (if a project is open it implies it exists, which is why there is no
need to check for `project.exists`).

Fix #1001714
(cherry picked from commit 784667c7b855cd603937b9a92a82c36c05341672)
Merge pull request #399 from dotta/backport/juno/issue/transitive-dependencies-should-not-consider-closed-projects-1001714

[backport 3.0.x-juno] Only consider opened projects when computing a project's direct dependencies