Sparse registries are not supported

[psmit]

Describe the bug
We use a private registry which is "sparse-only". As a result, cargo deny does not work anymore and returns an error indicating that the protocol is not supported.

To Reproduce
Steps to reproduce the behavior:

1. Add a sparse only registry to config.toml

[registries.myregistry]
protocol = "sparse"
index = "sparse+https://my-registry.domain/"

2. Run cargo deny check in a project that depends on this registry:

cargo deny check
2023-04-04 03:47:22 [WARN] unable to parse source url 'sparse+https://my-registry.domain/': parse error: unsupported source protocol: sparse
2023-04-04 03:47:22 [WARN] unable to parse source url 'sparse+https://my-registry.domain/': parse error: unsupported source protocol: sparse
2023-04-04 03:47:22 [ERROR] parse error: parse error: unsupported source protocol: sparse for key `package.source` at line 3956 column 1

Expected behavior
I expect cargo deny to not throw errors.