Prepare files for stage1 building

[lucab]

First, thanks a lot for this, it's great!

I've voiced for it to get more consideration by upstream, and we plan to actively use it for bootstrap purposes in Debian. I'm trying to use it for offline builds of stage1, and I can currently do it in a hack-ish way.

In particular, I'm trying to lay down all crates in a way that can be later re-used by ./configure+make. The current approach is to use paths overrides, but a better way would be to synthesize a registry index and point entries to local packages.

As such, my wishlist here is to steal some of the functionalities of cargo-vendor, in order to:

• resolve dependencies and download crates (probably already ok as-is in cargo-bootstrap)
• arrange paths so that they could be used by both carg-bootstrap and stage1 cargo builds
• generate a dummy registry pointing to local crates