abuild: only scan /lib /usr/lib, $rpath and $ldpath for provides

This is to avoid scan dlopen'ed plugins.

We scan any rpath set by any subpackage from same apkbuild. If it depends
on rpath to other package, developer will have to add that to ldpath.

This change means we have to move generation of .provides-so and .needs-so
til after all .rpaths are generated.