[MacPorts] #59538: github portgroup doesn't work with default master_sites from other portgroups

[MacPorts]

#59538: github portgroup doesn't work with default master_sites from other
portgroups
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  Reporter:  jmroot  |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:          |
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Comment (by ryandesign):

 A zillion ports override the github portgroups' master_sites. I haven't
 audited them to see how many of them rely on the fact that they can do so
 before calling github.setup. The intended use was that github.setup would
 be called near the top of the portfile, in place of the name and version
 lines, but many many ports adopted a different style in which github.setup
 is called in several different subports.

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Ticket URL: <https://trac.macports.org/ticket/59538#comment:1>
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