Virtual packages have the origin pointer set to NULL. Trying to print it
using the BLOB_PRINTF macros causes a segmentation fault.

Inspired by the `print_origin_name` function from `src/search.c` this
commit attempts to fix it by checking whether `pkg->origin` is NULL
before attempting to print it. If it is NULL the pkg name is printed
instead.

Since printing the pkg name requires a different format string this
commit splits the printf call for printing the package line into
multiple ones. The output format shouldn't have changed at all though.

list: use `apk list --providers` to search virtual providers instead of enabling virtuals by default

The list applet provides a convenient way of inspecting both the available
and installed package databases by listing their contents.  In some ways,
it is similar to `apk search` but is considered to be a superset of
`apk search` functionality.

A few `apk list` criterion are not yet ready though, such as `apk list --depends`
which searches by runtime dependency (replacing `apk info --rdepends`).