[refactor] clean up sass import chains

- simplifies import chains so we're not importing the same thing multiple times (e.g. when you `@import "archetype";`)
- makes sub-modules more standalone (e.g. `@import "archetype/util"` is more independent)
- this improves compile time (without `compass-import-once`)
- added helper methods to register/check modules: `register-archetype-module`, `has-archetype-modules`, `require-archetype-modules`