pubspec_parse: Added support for `executables` field in `pubspec.yaml` #279

Job	Run time
health / breaking / run breaking health check	1m 13s
health / coverage / run coverage health check	1m 39s
health / changelog / run changelog health check	1m 7s
health / do-not-submit / run do-not-submit health check	1m 8s
health / leaking / run leaking health check	1m 10s
health / license / run license health check	1m 7s
health / comment	4s
	7m 28s

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