[MacPorts] #56110: Travis: skip building ports from the obsolete PortGroup

[MacPorts]

#56110: Travis: skip building ports from the obsolete PortGroup
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  Reporter:  mojca          |      Owner:  l2dy
      Type:  defect         |     Status:  new
  Priority:  Low            |  Milestone:
 Component:  buildbot/mpbb  |    Version:
Resolution:                 |   Keywords:
      Port:                 |
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Comment (by l2dy):

 `mpbb` checks for "already been built" packages, which is not desired on
 Travis CI.

 So we should re-implement
 [https://github.com/macports/mpbb/blob/387064c27c616757509ec8462b72cfe512b228ad
 /mpbb-list-subports#L62-L76 the obsolete check] in `mpbot-github` or
 implement a switch in `mpbb` to disable this check and use `mpbb` for
 listing subports.

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Ticket URL: <https://trac.macports.org/ticket/56110#comment:1>
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