Adapt `package:messages` to be loading mechanism independent #750

Job	Run time
health / breaking / run breaking health check	59s
health / changelog / run changelog health check	53s
health / coverage / run coverage health check	1m 1s
health / do-not-submit / run do-not-submit health check	47s
health / leaking / run leaking health check	54s
health / license / run license health check	49s
health / comment	5s
	5m 28s

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