[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 ryandesign):

 portindexes are generated on the server, once per OS version/arch
 combination, and, if the user receives their ports tree via rsync, is
 distributed to the user when they `sudo port sync` or `sudo port
 selfupdate`. The portindex that is generated must therefore be suitable
 for all users of that OS version/arch combination, and cannot vary based
 on what might be already installed on the system.

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Ticket URL: <https://trac.macports.org/ticket/55293#comment:8>
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