[MacPorts] #20157: add `replaced_by` portfile key

[MacPorts]

#20157: add `replaced_by` portfile key
------------------------------+---------------------------------------------
 Reporter:  jmr@…             |       Owner:  jmr@…           
     Type:  enhancement       |      Status:  new             
 Priority:  Normal            |   Milestone:                  
Component:  base              |     Version:                  
 Keywords:                    |        Port:                  
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 If the `foo` Portfile contains:
 {{{
 replaced_by    bar
 }}}
 then when upgrade runs on foo, it should deactivate foo and install bar.
 This will allow both replacing one port with another, like scrollkeeper to
 rarian, and renaming of ports.

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Ticket URL: <http://trac.macports.org/ticket/20157>
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