Make `CargoCallbacks` more discoverable #1986

Job	Run time
minimal	12s
docs	11s
check-cfg	21s
msrv	4m 38s
quickchecking	24s
rustfmt-clippy	32s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 55s
test-book	6s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 57s
test-expectations (ubuntu-latest)	1m 6s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 52s
test-expectations (macos-latest)	1m 33s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	2m 47s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 16s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 27s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 42s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 54s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	2m 59s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 59s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	3m 29s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 54s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 17s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 15s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 12s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 3s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 48s
test (macos-latest, 9.0, 0, 0, 0)	1m 23s
success	1s
	56m 13s

Provide feedback