[MacPorts] #57947: Portfiles fail to parse if the official ports tree is not in sources.conf

[MacPorts]

#57947: Portfiles fail to parse if the official ports tree is not in sources.conf
--------------------+--------------------
 Reporter:  jmroot  |      Owner:  (none)
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  base    |    Version:  2.5.99
 Keywords:          |       Port:
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 Problem introduced by [5e17a692f991a55a7f450e40ce47ab6fe0f2da57/macports-
 base]. Causes test failures:
 https://build.macports.org/builders/base-10.14_x86_64/builds/21/steps/test/logs/stdio

 Will also affect anyone using only a custom ports tree. Prior to this
 change, port would work without the official tree being in sources.conf;
 portfiles just wouldn't have access to the portgroups and mirror sites
 defined there.

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Ticket URL: <https://trac.macports.org/ticket/57947>
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