[MacPorts] #63680: glib2: implement new segregated port scheme, with separate x11/quartz subports, to support multiple versions installed side-by-side

[MacPorts]

#63680: glib2: implement new segregated port scheme, with separate x11/quartz
subports, to support multiple versions installed side-by-side
-------------------------+----------------------
 Reporter:  mascguy      |      Owner:  mascguy
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:  segregation  |       Port:  glib2
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 Given the challenges updating `glib2` - there are a tremendous number of
 dependent ports, requiring a corresponding amount of integration testing,
 headaches, etc - it's time to consider a segregated scheme, in the spirit
 of `boost`.

 The goal is to keep our two legacy ports - `glib2` and `glib2-devel` - as-
 is. This avoids any disruptions, breakages, etc.

 But for those ports that require the latest upstream version, they'll be
 able to utilize the new scheme. And since X11 and Quartz will be supported
 by separate non-conflicting subports, that will further reduce headaches.

 This work dovetails with eventual segregation of GTK-related ports, with a
 similar X11/Quartz subport strategy; tracked by long-standing ticket:
 issue:27990

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Ticket URL: <https://trac.macports.org/ticket/63680>
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