[MacPorts] #55293: set ${prefix} in portindex

[MacPorts]

#55293: set ${prefix} in portindex
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  Reporter:  RJVB         |      Owner:
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by RJVB):

 So how does that finagle out for build variants (those which modify the
 dependency list)?

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Ticket URL: <https://trac.macports.org/ticket/55293#comment:9>
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