Detect "SPDX Short Identifier" tags #4301

[pombredanne]

This PR adds support for a new, not-yet-supported variant for SPDX-License-Identifier which is "SPDX Short Identifier" and is used in the wild.

Reference: #4301

Fixes #4301

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)

[AyanSinhaMahapatra]

Thanks @pombredanne
There are some type errors at https://dev.azure.com/nexB/scancode-toolkit/_build/results?buildId=16220&view=logs&j=e968ac97-6fe5-5e03-7848-d38fdbd63e89&t=68b960ee-accc-5f66-8291-47be569f7afb&l=11639, looks great otherwise.