[MacPorts] #56023: obsolete portgroup: don't require replaced_by to be set before including the portgroup

[MacPorts]

#56023: obsolete portgroup: don't require replaced_by to be set before including
the portgroup
------------------------+----------------------
 Reporter:  ryandesign  |      Owner:
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  obsolete
------------------------+----------------------
 The obsolete-1.0 portgroup currently requires `replaced_by` to be set
 before including the portgroup. Don't require that; it's weird. Instead,
 maybe use an `option_proc` to monitor changes to `replaced_by`.

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Ticket URL: <https://trac.macports.org/ticket/56023>
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