rustpkg: Do not guess version if not given

rustpkg accessed git repo to read tags and guess package version,
but it's not quite useful: version can be given explicitly by user,
and implicit guess may cause confusions.
rustpkg::version: Remove enum Version

Currently rustpkg doesn't use SemanticVersion or Tagged, so they are
removed. Remaining variants are replaced by `Option<~str>`.
rustpkg: Compute hash to find crate

Previously rustpkg tried to parse filenames to find crate. Now ue use
deterministic hashes, so it becomes possible to directly construct
filename and check if the file exists.
rustpkg::crate_id: Remove CrateId

There is no significant difference between `rustpkg::crate_id::CrateId`
and `syntax::crateid::CrateId`. rustpkg's one is replaced by syntax's
one.
auto merge of #11478 : klutzy/rust/rustpkg-crate-id, r=cmr

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