Complete ECMA-427 2nd Edition documentation for submission to Ecma

[mjherzog]

This issue is for tracking completion of ECMA-427 documentation to be submitted to TC54 by the end of August. The changes will be submitted as PRs in https://github.com/Ecma-TC54/ECMA-427 GitHub repo.

The sections of ECMA-427 are:

• About - does not appear to require updates other than edition and date in the title
• Introduction - review for possible improvement (not normative)
• Clause 1: Scope - no change
• Clause 2: Conformance - no change
• Clause 3: Normative References - planning to add one reference for ABNF grammar
• Clause 4: Overview - could be improved/extended (not normative)
• Clause 5: Package-URL specification - changes
• Clause 6: Package-URL Type Definition Schema - changes
• Annex A: (normative) PURL Type Definition - refresh with updated schema content
• Annex B; (informative) Recommended Qualifiers Keys - new
• Annex C: (informative) ABNF Grammar - new
• Bibilography - add reference for ABNF grammar
• Colophon: No change
• Copyright & Software License: No change

NB: "Clause" is the Ecma terminology for the primary content sections of an Ecma standard dcoument