Finish implementing all rustpkg commands

[catamorphism]

do, fetch, info, prefer, test, uninstall, unprefer

[catamorphism]

Sub-bug of #5677

[graydon]

visiting for triage. I don't think 'prefer' and 'unprefer' have a role in this space. the others, might as well. I imagine fetch and test are the more important two.

[graydon]

nominating for feature completeness.

[catamorphism]

De-nominating, not the highest-priority bug for rustpkg

[alexcrichton]

Closing in favor of #11859