Make interface members + downstream types internal #423

Job	Run time
test-netstandard	45s
test-unity (2021.3.27f1)	3m 6s
test-unity (2022.3.2f1)	3m 42s
inspectcode	1m 39s
test-mutation-core / stryker	3m 57s
test-mutation-gherkin / stryker	3m 0s
	16m 9s

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