This PR makes `event_timeout` and `event_timeout_ztimer` two distinct
pseudomodules, where the only api difference is in the init function.
If only `event_timeout_ztimer` is selected then no default ZTIMER
backend is selected and the old init function is not implemented.
If only `event_timeout` is selected then `xtimer` is used unless
`ztimer_usec` is included. In which case the `xtimer` wrapper on top
of `ztimer` is used and `xtimer` is not directly selected. This
allows for the legacy api to be supported with `ztimer_usec` as
a drop-in replacement.
If `event_timeout` and `event_timeut_ztimer` are selected then
`event_timeout` SRC file is excluded from compilation.
