remove special cases from depth first search

- regular depth-first search already handles circular dependencies and
  hitting the same package twice
  - the special cases might have saved a teensy amount of work, but the
    simplification is preferable

- pre-release handling here was not consistent with code elsewhere, and
  since "elsewhere" happens first, it wins
  - given a constraint like ">=1.0" the removed code would have allowed
    1.0rc1, but elsewhere in the codebase constraints are taken to mean
    what they say