Add a termination condition when a -to package is provided

If a `-to` package is specified, and `-all-paths` is not, then the query can be sped up by adding a termination condition to the dependency graph construction. See https://github.com/google/godepq/blob/e3ed13bdd8d1fb68ac409813fb36fcbcbd89cbae/pkg/deps.go#L33
