Optimize repoman visibility checks to access as little

Optimize repoman visibility checks to access as little
metadata as possible. (improves performance especially
in cases where metadata needs to be generated). This
works by starting at the lowest version since that's
most likely to have keywords and it returns as soon as
the first visible package is found. (trunk r8309)

svn path=/main/branches/2.1.2/; revision=8369