Sbom-tool generates cargo purl strings with extra "/" char

[henrylyons]

The sbom-tool generates cargo purl references with an extra "/", for crates pulled from the default repository (https://crates.io). Example: pkg:cargo//[email protected]

This behavior does not repro for other package managers, such as NuGet. Also, the cargo purl strings do not conform to the purl specification: https://github.com/package-url/purl-spec/blob/main/PURL-TYPES.rst