alternatives: special-case busybox as alternatives provider

alternatives: special-case busybox as alternatives provider

Almost all busybox applets are alternatives to some other existing
"full" utilities.  To lift the maintenance burden of enumerating CONFIG
symbols, symlink path of each applet, we special case busybox here as a
known alternatives provider.

All file pathes provided by busybox will serve as fallback alternatives
with -inf priority.  Packages intending to switch to using alternatives
mechanism will also not need to depend on the same kind of change be
applied on busybox in base system

v3 <- v2 continue to next when fetching provider pkg failed
v2 <- v1 Move busybox-specific part into data section

Signed-off-by: Yousong Zhou <zhouyousong@yunionyun.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>