Add back the selfupdate command (interim mode) #501

Job	Run time
CI test (make validate)	37s
Code coverage (codecov)	33s
Build moodle-plugin-ci.phar	49s
Integration tests (8.2, master)	9m 9s
Integration tests (8.2, MOODLE_403_STABLE)	6m 17s
Integration tests (8.2, MOODLE_402_STABLE)	6m 23s
Integration tests (8.1, MOODLE_401_STABLE)	7m 41s
Integration tests (8.0, MOODLE_400_STABLE)	9m 4s
Integration tests (8.0, MOODLE_311_STABLE)	6m 51s
Integration tests (7.4, MOODLE_310_STABLE)	6m 5s
Integration tests (7.4, MOODLE_39_STABLE)	6m 14s
Integration tests (7.4, MOODLE_38_STABLE)	6m 32s
Integration tests (PHAR) (8.2, master)	6m 14s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	5m 55s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	7m 26s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	7m 41s
	1h 33m 31s

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