cargo-bazel: Corrected sparse registry splicing code

[JulianSchmid]

This is a fix for supporting sparse-registries (private).

Issue:

When we tried to use crate_universe together with an internal sparse registry (Artifactory) we ran into an issue. cargo-bazel splice was not correctly detecting sparse registries as "sparse", even though the URLs correctly started with the "sparse+https://" prefix in the Cargo.lock file. Instead it re-interpreted them as registries with a git index.

The root cause of this is thatcargo_lock::Lockfile automatically removes the prefix sparse+ from the URL and the only way to determine that a registry is sparse is to check the kind. The code previously assumed it could check for sparse registries by checking if the url starts with "sparse+https://".

Problem solution:

This pull requests adapts the code so it uses the lockfile "SourceKind" instead of the URL to check if a registry is sparse or not.

[illicitonion]

Thanks!