Description

When the --distro flag is used, we update metadata.OS, but the PURLs for OS packages remain unchanged.

This leads to two problems:

1. OS initially not detected
   If the OS is not detected initially, we keep the package PURLs empty even after overriding the OS via --distro.
   As a result, VEX files cannot be used correctly (see: VEX pkg:identifier fails, because SBOM seems to be missing PURL when --distro alma/9 override is used #9767).
2. OS initially detected
   If the OS is detected initially, there will be a mismatch between metadata.OS and the package PURLs (their namespace and/or distro qualifier).

Proposed solution

After overriding the OS we should recompute and update the PURLs for OS packages, since OS information is part of the PURL itself.

Relevant code:

Discussed in #9767