Set debian namespace from clues

[AyanSinhaMahapatra]

We will now deduce and populate debian distro information, i.e. their namespace from clues present in version/maintainer info.

Reference: https://github.com/nexB/scancode.io/issues/899

Tasks

• [x] Reviewed contribution guidelines
• [x] PR is descriptively titled 📑 and links the original issue above 🔗
• [ ] Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR Run tests locally to check for errors.
• [x] Commits are in uniquely-named feature branch and has no merge conflicts 📁
• [x] Updated documentation pages (if applicable)
• [x] Updated CHANGELOG.rst (if applicable)