Refactor generated spec data to facilitate looking up by spec name

[westonruter]

Summary

Fixes #4597.

Todo

• There should be an efficient way to look up a spec by a spec name regardless of the tag it occurs in.
• Instead of looping over the tag specs to find the one with the matching spec name, it would be better if the spec name was the array key.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[westonruter]

• There should be an efficient way to look up a spec by a spec name regardless of the tag it occurs in.

This is the best possible right now:

	/**
	 * Get tag by spec name.
	 *
	 * @since 1.5
	 *
	 * @param string $spec_name Spec name.
	 * @return array|null Allowed tag spec, or null if the tag does not exist.
	 */
	public static function find_tag_spec( $spec_name ) {
		foreach ( self::$allowed_tags as $tag_name => $allowed_tag ) {
			foreach ( $allowed_tag as $rule_spec ) {
				if (
					( ! isset( $rule_spec['tag_spec']['spec_name'] ) && $tag_name === $spec_name )
					||
					( isset( $rule_spec['tag_spec']['spec_name'] ) && $rule_spec['tag_spec']['spec_name'] === $spec_name )
				) {
					return array_merge( compact( 'tag_name' ), $rule_spec );
				}
			}
		}
		return null;
	}

We can eliminate looping over $allowed_tag by using the spec name as the array key. But then, ideally we wouldn't have to loop over self::$allowed_tags.

Only way I can think of to prevent this is having a separate index of spec_name mapping to tag/index.

[westonruter]

Linking to the relevant docs page on amp.dev for each violated tag spec depends on each tag spec having a spec_url defined. See ampproject/amphtml#26033.

[westonruter]

Closing in favor of the work that @schlessera is doing in #4566.