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
Merge pull request #388 from dotta/issue/transitive-dependencies-should-not-consider-closed-projects-1001714

Only consider opened projects when computing a project's direct dependen...