Add new --purl option to only get purls

[AyanSinhaMahapatra]

Adds a new option --purl to only parse and return in package data the purl fields, in package scan.

Reference: #3464

Tasks

• Reviewed contribution guidelines
• 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.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Looked for possible updates in documentation and added updates if applicable
• Updated CHANGELOG.rst

[pombredanne]

Could it be simpler not to introduce a purl_only=False arg everywhere and instead just to the purl-only thing on the output of the parse?

[AyanSinhaMahapatra]

Closing this in favor of the updated implementation at: #3689
This is:

• simpler code, easier to maintain and has fewer changes
• also has other metadata with purl, but does not still perform license/copyright detection, so just as fast.