gc: don't visit implicit referrers on garbage collection

Before the change garbage collector was not considering
`.drv` and outputs as alive even if configuration says otherwise.
As a result `nix store gc --dry-run` could visit (and parse)
`.drv` files multiple times (worst case it's quadratic).

It happens because `alive` set was populating only runtime closure
without regard for actual configuration. The change fixes it.

Benchmark: my system has about 139MB, 40K `.drv` files.

Performance before the change:

    $ time nix store gc --dry-run
    real    4m22,148s

Performance after the change:

    $ time nix store gc --dry-run
    real    0m14,178s

0 files changed, 0 insertions, 0 deletions