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

src/libstore/gc.cc

@@ -678,7 +678,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                 alive.insert(start);
                 try {
                     StorePathSet closure;
-                    computeFSClosure(*path, closure);
+                    computeFSClosure(*path, closure,
+                        /* flipDirection */ false, gcKeepOutputs, gcKeepDerivations);
                     for (auto & p : closure)
                         alive.insert(p);
                 } catch (InvalidPath &) { }