rustpkg: CrateId cleanup

This patchset consists of three parts:

- rustpkg doesn't guess crate version if it is not given by user.
- `rustpkg::version::Version` is replaced by `Option<~str>`.
  It removes some semantic versioning portions which is not currently used.
  (cc #8405 and #11396)
  `rustpkg::crate_id::CrateId` is also replaced by `syntax::crateid::CrateId`.
- rustpkg now computes hash to find crate, instead of manual filename parse.

cc @metajack