[package_config] Remove support for `.packages` files. #403

Job	Run time
health / breaking / run breaking health check	1m 6s
health / changelog / run changelog health check	1m 12s
health / coverage / run coverage health check	1m 11s
health / do-not-submit / run do-not-submit health check	1m 15s
health / license / run license health check	1m 7s
health / leaking / run leaking health check	1m 0s
health / comment	3s
	6m 54s

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