fix: restore stoppingEvent/stoppedEvent helpers for plugin stop hook

The stop lifecycle hook added in 672dc14d2 calls stoppingEvent and
stoppedEvent from pkg/compose/plugins.go, but the helpers had been
removed by the earlier wrapper-cleanup in da530c723. The result was a
broken go build for pkg/compose on upstream/main.

Restore the two helpers alongside the symmetric creating/created and
removing/removed pairs so call sites in plugins.go match the existing
pattern used in the same switch.

Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>