Measure package assembly timing with CLI option

[pombredanne]

This PR adds a new option to time package scans to support:

• fast-scan: Create scan benchmark for package detection in ScanCode Toolkit #4061 (comment)

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]

@AyanSinhaMahapatra is this something you want to merge?