[MacPorts] #50966: Qt5 PortGroup files; preparing for port:qt5-kde and the KF5 port family

[MacPorts]

#50966: Qt5 PortGroup files; preparing for port:qt5-kde and the KF5 port family
---------------------------+----------------------
  Reporter:  RJVB          |      Owner:  mkae
      Type:  update        |     Status:  assigned
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:
Resolution:                |   Keywords:
      Port:  qt5, qt5-kde  |
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Comment (by RJVB):

 The `qt5.depends_component` proposal has been addressed, the other points
 remain open. Qt5-kde will still require its own portgroups, and a
 mechanism for ports to indicate that they prefer this port.

 Given how things have worked out I now lean towards a solution where we
 have only 2 Qt5 PGs but also 2 qmake5 PGs, each of which transfers control
 to the other when that should be done. IOW, qt5-1.0 and qmake5-1.0
 transfer control to qt5-kde-1.0 or qmake5-kde-1.0 if it turns out that
 port:qt5-kde is installed, and vice-versa (when port:qt5 is installed).
 This implementation means we're really independent in the internal
 implementation details, and only need to ensure that the control transfer
 happens reliably and at an appropriate point in the file. It would also
 mean that ports can indicate their preference simply by including the
 qt5-kde PG directly and will have to take additional action if they need
 to depend on qt5-kde only for some reason.
 You probably saw the sample implementation I posted elsewhere
 (qmake5-kde-1.0.tcl and the 4 lines (7, with the comments) added to
 qmake5-1.0.tcl). It's working nicely.

 You know I have tried to discuss the approach I've followed with the
 preference mechanism (after Marcus mentioned he thought it was in
 violation of certain principles), and as usual didn't get very far:
 - Clemens officially doesn't care because the explanation is too long
 (that's my interpretation of his reaction but he hasn't disagreed with it
 ;))
 - Larry hasn't disagreed with anything, yet
 - Mojca seems support it, or at least an approach that is so similar I
 haven't seen any differences with what I do.

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Ticket URL: <https://trac.macports.org/ticket/50966#comment:6>
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