Package foo needs bar

Rule: if foo is compiled against libbar.so.0.0, then 
it requires libbar.so.0.* to work.

If we update bar-0.0 to bar-1.0, we still need to 
have libbar.so.0.0 for foo to work.

@lib declares shared libraries.
@wantlib records shared library dependencies
pkg_create writes @wantlib from WANTLIB
pkg_add builds a network of libraries on the fly.
after an update, we have .libs-bar-0.0