[MacPorts] #59839: Record default python and perl versions

[MacPorts]

#59839: Record default python and perl versions
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 Reporter:  mf2k         |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:
 Keywords:               |       Port:
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 I'd like for base to keep track of the currently accepted default version
 of important programs like python and perl. Then Portfiles could refer to
 that for the purpose of setting dependencies. Also, it would also be nice
 for variants.conf to be able to override those values.

 While specific versions of python, in the past, have been important (e.g
 python 2 vs. 3), I don't think that is an issue much anymore. Where it is
 an issue, then Portfile in question can continue to hardcode the version
 needed.

 Base itself should not hardcode the default versions and they should be
 easy to update anytime like allowed Xcode versions.

 Examples:
 {{{
 default_python python37
 default_perl perl5.30
 }}}

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Ticket URL: <https://trac.macports.org/ticket/59839>
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