Identify better ways to use cargo

[ibabushkin]

We currently have a number of issues related to our use of cargo that all manifest as some sort of advanced cargo feature interfering with our (rather hacky) way of talking to cargo.

The currently prominent issues: #67, #91. Also, best-effort integration with cargo's newly stabilized offline mode would be quite useful at times.

We have multiple ways of addressing these -- either update our interaction with cargo to support all these features cleanly, or approach the issues from a different angle, for example using something like escargot and cargo-metadata which we already make use of. The main drawback of such an approach would be that it's probably quite hard to get all the features we need from outside cargo. At the same time, not depending on cargo has its benefits, as long as our implementation for, say, finding build artifacts is stable across cargo versions. Most importantly, changes to cargo will no longer cause as much breakage to semverver as in the past.