rustpkg probably won't check for upstream updates

[catamorphism]

If you do:

extern mod foo = "github.com/bar/quux";

then rustpkg won't check for updates to the quux repo after the first time, since the locally cached sources won't have changed. Arguably this behavior should be different, and workcache's freshness functions should take the state of a remote repo (if one exists) into account.

[catamorphism]

Also, it should handle the case where there's no network connection gracefully (just don't download patches, as opposed to erroring out).

[emberian]

rustpkg is gone.