Reimplement portdbapi.visible() so that is works without

Reimplement portdbapi.visible() so that is works without
using self.xmatch(). This should be 100% compatible with
the previous implementation and have comparable
performance. By eliminating the xmatch() dependency, the
code becomes more generically usable, like for
visibility filtering of binary packages. It seems that
it will be necessary to move this code outside of the
dbapi since visiblity filtering is really a resolver
level function and to solve things like bug #88613 will
require a more flexible approach. (trunk r7892)

svn path=/main/branches/2.1.2/; revision=7925