Support for Dependency Ranges

[DavePearce]

Currently, all dependencies must be given explicitly (e.g. std="0.2.5"). Should be able to give ranges (e.g. std="0.2.*"). This raises a number of issues though:

• Resolution. Need to then figure out an algorithm for dependency resolution given a graph. Obvious approach is some kind of greedy approach.
• Lock File. Need support for wy.lock file which is similar to cargo.lock. Specifically, identifies which concrete versions were actually used.

The rough idea would be that the package list is only updated when no lock file already exists.