Normalize the contents of `extension.json` to provide consistent access

[coveralls]

Pull Request Test Coverage Report for Build 45

• 133 of 139 (95.68%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 95.703%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ocdsextensionregistry/extension_version.py	67	73	91.78%
<!--	Total:	133	139	95.68%	-->

Totals
Change from base Build 39:	-0.3%
Covered Lines:	245
Relevant Lines:	256

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💛 - Coveralls