Add command for downloading a new benchmark

[Kobzol]

Inspired by Zulip conversion between @nnethercote and @Mark-Simulacrum.
Usage:

$ ./collector download crate syn 1.0.0 --name syn2
$ ./collector download git https://github.com/dtolnay/syn

[nnethercote]

Is it necessary to be able to get the crate by cloning from github? That's what I was doing and this morning @lqd explained how to download from crates.io and my experience so far is that the crates.io way is simpler and less error-prone.

[Kobzol]

Sure, but I suppose that not all crates are on crates.io? For example people that create performance regression issues on the rustc repository often link their github repo.

[Kobzol]

I changed the logic to guess the unpacked package name and added a comment.

[nnethercote]

It occurs to me that if #1181 is going to require a primary or secondary entry in the perf-config.json, then this command should probably create that file. Possibly requiring primary or secondary as an additional argument. If this is merged first, I guess that could be added in #1183.

[Mark-Simulacrum]

Yeah, I agree that creating that file would be good.

I'm not sure why CI is failing, but that seems like the main blocker left here.

[Kobzol]

@nnethercote You are completely right :) I just didn't add it here since I wasn't sure which PR would get merged first.

CI seems broken since some package is missing, I'm not sure if its transient or not 🤷 Maybe the workflow will need updating.

[Kobzol]

The package still returns 404, so we might need to update package versions on CI.

[Kobzol]

Rebased over master with fixed CI.

[nnethercote]

Looks good, just a couple of minor suggestions.