Add lazy sync support for Rust packages
Created by: olafurpg
Previously, Sourcegraph returned 404 when visiting the correct URL of an Rust package with a concrete version that wasn't tracked. This commit fixes that issue so that Sourcegraph lazy syncs the package+version and returns a 200 instead of 404.
This already worked for other package hosts like npm/pypi, this PR only adds the same support for Rust packages.
Fixes https://github.com/sourcegraph/sourcegraph/issues/37921
Test plan
The change was manually tested with the command
SOURCEGRAPHDOTCOM_MODE=true sg start
and then visiting the URL of a crate that has not been tracked locally, for example https://sourcegraph.test:3443/crates/[email protected]