replaced_by question

[Jeremy Lavergne]

Why don't we block the installation of ports that have replaced_by set?

http://trac.macports.org/changeset/57005

I just wrote our first replaced_by and it shows up in port outdated if  
it's revision 1 you have installed and run upgrade, but you can  
install this revision 2 and then it never shows up as outdated and  
won't get replaced.

How about the following ideas:
  * anytime replaced_by is found, it's epoch_override is turned on
  * if installing a port that has replaced_by, fail (or just switch it  
out for the replacement)