[MacPorts] #62807: openmpi/mpich: refactor duplicated logic, and simplify ports, via new portgroup

[MacPorts]

#62807: openmpi/mpich: refactor duplicated logic, and simplify ports, via new
portgroup
----------------------------+----------------------
  Reporter:  mascguy        |      Owner:  mascguy
      Type:  enhancement    |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:
Resolution:                 |   Keywords:
      Port:  openmpi mpich  |
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Comment (by kencu):

 we do need to keep the older gcc versions for other purposes, and probably
 the older clangs too for similar reasons (bootstrapping or certain older
 code that needs it and can't/won't be updated).

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Ticket URL: <https://trac.macports.org/ticket/62807#comment:8>
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