Data Package versioning and profiles simplification

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Overview

It's a meta issue that includes these issues:

• Profile is a url rather than string #689
• Adding a version to profile attribute (or doing this via a "spec" namespace) #444
• Don't require Tabular Data Package to have profile = tabular-data-package #743
• Separate concerns between metadata extensions and resource types #887

All of them are very coupled and hopefully will be resolved by one internally consistent solution (currently drafted at #42)

[roll]

SPLIT on smaller issues