[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):

 One example would be with port families like python, perl or Qt5 which all
 share (are served by) a common PortGroup. If that PortGroup or the
 Portfiles need to do something that depends on what the user has installed
 it is a lot easier (and probably faster) to check for the presence of an
 installed file that to query the registry. (`file exists` is a Tcl
 primitive and doesn't need catching.)

 One application would be port:qt5 vs. my pending port:qt5-kde . These have
 partial PortGroup sharing, but the main payload sits in dedicated PG files
 which are loaded as a function of what is installed.

 It would be trivial to work around `$prefix` not being declared in/by
 portindex, but not elegant. Is there a solid reason why `$prefix` and
 possibly other useful and often-used variables aren't set when portindex
 is run?

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Ticket URL: <https://trac.macports.org/ticket/55293#comment:3>
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