Check type of PURL

The following PURL is considered as valid by purl-validator: `pkg:unknown/package@1.0.0`

I would expect that the validator checks the type (it should be one of alpm apk bazel bitbucket bitnami cargo cocoapods composer conan conda cpan cran deb docker gem generic github golang hackage hex huggingface julia luarocks maven mlflow npm nuget oci pub pypi qpkg rpm swid swift)

