Becuase it is already done by __apt_source.
s/-/_/ because the explorers are following an other convention :-)
This type now automatically calls the type __apt_update_index to update
the package index if something changed.
The manpage now lists all OSes where this type supports backports.
As discussed in the chat, this type now supports a broader list of OSes
which it supports backports for. Because of this, it was renamed to
something more generic. "apt" should fit in.