[MacPorts] #65212: opencv4: contrib variant: fetch additional artifacts via portfile

[MacPorts]

#65212: opencv4: contrib variant: fetch additional artifacts via portfile
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 Reporter:  mascguy      |      Owner:  mascguy
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.7.2
 Keywords:               |       Port:  opencv4
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 Presently, additional contrib-related artifacts are implicitly fetched via
 the build files, rather than explicitly via the portfile.

 While this doesn't cause any issues - and has been the case for a long
 time - it would be better to explicitly perform the fetch ourselves.
 Indeed, that's what's being done for `opencv3`, and we should do the same
 for `opencv4`.

 With this change, the additional artifacts will be archived and published
 by MacPorts, ensuring builds aren't dependent on upstream resources.

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Ticket URL: <https://trac.macports.org/ticket/65212>
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