Possible refinements to version handling

[catamorphism]

On the mailing list, @z0w0 suggested:

• inferring a version from either a tarball filename or a directory name (e.g. foo-0.1.tar.gz) if no version is available from VCS
• allowing link_meta to supply a version if it's there, though never requiring it

I think these ideas have both come up when I was talking to @graydon about it but I don't think there's another issue mentioning either. Also, I think the second one may be what rustpkg already does, though there's no test case.

[catamorphism]

Meta-bug: #5677

[thestinger]

triage bump

[alexcrichton]

rustpkg has been removed