[MacPorts] #63053: macportsbot: detect invalid maintainer GitHub handles

[MacPorts]

#63053: macportsbot: detect invalid maintainer GitHub handles
----------------------------+---------------------
 Reporter:  chrstphrchvz    |      Owner:  admin@…
     Type:  enhancement     |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  server/hosting  |    Version:
 Keywords:                  |       Port:
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 Maintainers are not notified by macportsbot if they have changed their
 GitHub handle (or if they deleted their GitHub account, but in that case
 the maintainer would probably have to be emailed). I have noticed this
 multiple times, but I believe it is unlikely that others notice this
 situation (the only indication is that `@handle` in macportsbot’s comment
 isn’t shown in boldface), so it would help if macportsbot detects this and
 says something.

 It is usually possible to retrieve the maintainer’s new handle manually,
 such as by navigating the blame feature on GitHub and/or looking for past
 PRs made by that user. It may be possible to automate this, for example if
 macportsbot caches user IDs for valid handles (e.g. 7941193 for
 @chrstphrchvz) and then looks up the new handle from the user ID if it
 becomes invalid.

 (Yes, I understand that it is likely maintainers’ responsibility to ensure
 their contact info in portfiles is up-to-date.)

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Ticket URL: <https://trac.macports.org/ticket/63053>
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