add pkg scheme to npm generic file PURLs

[taylormadore]

The pkg scheme is required for each component according to the PURL specification.

Maintainers will complete the following section

• [x] Commit messages are descriptive enough
• [x] Code coverage from testing does not decrease and new code is covered
• [n\a] New code has type annotations
• [n\a] OpenAPI schema is updated (if applicable)
• [n\a] DB schema change has corresponding DB migration (if applicable)
• [n\a] README updated (if worker configuration changed, or if applicable)
• [ ] Draft release notes are updated before merging

[eskultety]

LGTM