[MacPorts] #56606: port -D install doesn't work for subports

[MacPorts]

#56606: port -D install doesn't work for subports
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  Reporter:  wyuenho  |      Owner:  (none)
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:
 Component:  base     |    Version:  2.5.1
Resolution:           |   Keywords:
      Port:           |
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Comment (by jmroot):

 There's no way for the program to know which subport you want, unless you
 indicate it somehow. Specifying a port name in the command is the usual
 way this is done, but it only works if the name is in the PortIndex. If
 you have to work with unindexed ports for some reason, you can append
 `subport=something` to the command.

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Ticket URL: <https://trac.macports.org/ticket/56606#comment:1>
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