Add a faster package scan with --package-only

[AyanSinhaMahapatra]

Adds a new command line option --package-only:

• Faster package metadata detection in files
• Skips License/Copyright detection from extracted license/copyright statements

This replaces the previous implementation at #3525

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 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

[pombredanne]

Looking good! ... please merge and add some further tests later.

[pombredanne]

Thanks. Good to merge... you may want to refine the help text for the option: metadata, steps, codebase are words we are not using often so they may confuse users.

[AyanSinhaMahapatra]

Merging, thanks!