Ability to hint at a custom distro

Idea from @samj1912 during the latest community meeting

**What would you like to be added**:

The ability to hint at what the distro is instead of having syft detect it directly. This is most impactful during pURL creation for OS-level packages.

**Why is this needed**:

In the cases where the distro is incorrectly identified (and the distro flavor can't be easily identified) then the pURLs for packages are affected.

The issue does not name files or tests. Start by tracing how syft detects the operating-system distribution and how that value is used during pURL creation for OS-level packages. Define the supported hint input and verify that hinted distributions produce the intended package pURLs without breaking automatic detection.

Written by the indexing model from the issue text.