[MacPorts] #65721: app portgroup: create new 1.1 version, utilizing callback mechanism

[MacPorts]

#65721: app portgroup: create new 1.1 version, utilizing callback mechanism
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  Reporter:  mascguy  |      Owner:  mascguy
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:  2.7.2
Resolution:           |   Keywords:  portgroup
      Port:  gramps   |
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Comment (by mascguy):

 Replying to [comment:13 mascguy]:
 > Unless someone can provide a strong argument against it, I'm going to
 add an `app 1.1` pg using a callback.

 For anyone interested in my approach, this is what I'm thinking:
 * Refactor minimally, to ensure it's as close to `app 1.0` as possible.
 Also reduces chance of introducing any bugs.
 * Include a backward-compatibility option, which disables use of callback.
   * Provides a migration path for existing ports.
   * Would also allow us to ultimately fold the changes back into `app
 1.0`, avoiding the need for a new version of the pg.
   * But if we do fold back into `app 1.0`, that option would be enabled by
 default.
 * Implement some type of unit-testing approach, to automate validation of
 error cases. Not strictly necessary, but would be nice!

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Ticket URL: <https://trac.macports.org/ticket/65721#comment:16>
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