[MacPorts] #62011: opencv/opencv4: eliminate portfile duplication, via subport

[MacPorts]

#62011: opencv/opencv4: eliminate portfile duplication, via subport
-----------------------------+----------------------
  Reporter:  mascguy         |      Owner:  mascguy
      Type:  enhancement     |     Status:  assigned
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:
Resolution:                  |   Keywords:
      Port:  opencv opencv4  |
-----------------------------+----------------------

Comment (by mascguy):

 This will be done in phases, coordinated with reinstatement of OpenCV 2
 port (issue:49670):
 1. Add variants missed during opencv/opencv4 reconciliation (opencv
 +openmp, opencv4 +openni).
 1. Add new port `opencv3`, segregated like opencv4. Also include initial
 changes, supporting later conversion to subport.
 1. Update ports presently dependent upon `opencv`, to use `opencv3`. And
 update port `opencv` to be a stub port, with `replaced_by opencv3`, etc.
 1. Add new port `opencv2`, using same segregation strategy, etc.
 1. Reconcile all three into single `opencv` port, with three subports.

 Thoughts/feedback welcome, as always.

-- 
Ticket URL: <https://trac.macports.org/ticket/62011#comment:3>
MacPorts <https://www.macports.org/>
Ports system for macOS