[MacPorts] #56025: Support ignoring license lmitations in port mirror

[MacPorts]

#56025: Support ignoring license lmitations in port mirror
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 Reporter:  neverpanic   |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Low          |  Milestone:  MacPorts Future
Component:  base         |    Version:
 Keywords:               |       Port:
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 https://github.com/macports/macports-base/pull/59 added exclusions to
 `port mirror` without a way to ignore these exclusions, which may be
 desirable when running a private mirror.

 Document that this exclusion is enabled starting with MacPorts 2.5.0 and
 implement the `--ignore-license` flag in `port mirror`.

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Ticket URL: <https://trac.macports.org/ticket/56025>
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