Improve SPDX import from Python SBOMs

**Describe the bug**
A clear and concise description of what the bug is.

I run a the `load_sbom` pipeline using this input
https://www.python.org/ftp/python/3.13.9/Python-3.13.9.tgz.spdx.json

The SBOM has a lot of problems including:
- listing each and every file in relationships to embedded packages which creates an SBOM of 93,000+ lines. Not sure we can do much about that
- the lack of package type and PURLs for some generic packages, that we then report as "unknown" type, like with "pkg:unknown/expat@2.7.3" when we could do better especially when there are download URLs and we could instead create a proper "generic" PURL
- pip vendored dependencies are reported but not that they are patched or the download URL may be misleading.... I would be surprised that Python 3.13's pip 25.2 depends on a msgpack binary wheel built for macOS.

Overall there are issues in the SPDX, but there are also issues on what we do with it.

For reference, attached are the SPDX, its export to CycloneDX from SCIO and also its re-export to SPDX from SCIO:

[Python-3.13.9.tgz.spdx.json](https://github.com/user-attachments/files/23022087/Python-3.13.9.tgz.spdx.json)
[scancodeio_sbom-round-trip_results-2025-10-21-12-59-02.cdx.json](https://github.com/user-attachments/files/23022089/scancodeio_sbom-round-trip_results-2025-10-21-12-59-02.cdx.json)
[scancodeio_sbom-round-trip_results-2025-10-21-13-44-58.spdx.json](https://github.com/user-attachments/files/23022088/scancodeio_sbom-round-trip_results-2025-10-21-13-44-58.spdx.json)

**System configuration**
- Which version of ScanCode.io are you running? 35.4.0
- Are you running the app using Docker? yes
- On which OS? linux
- What inputs are you using? see above
- Which pipeline are you running? load_sbom