[MacPorts] #55936: Support tags and multiple values for dist_subdir

[MacPorts]

#55936: Support tags and multiple values for dist_subdir
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 Reporter:  ryandesign   |      Owner:  ryandesign
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:  MacPorts Future
Component:  base         |    Version:
 Keywords:               |       Port:
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 Just as MacPorts base supports multiple values for `distfiles` and
 `master_sites`, and allows tags to be used to link a specific distfile
 with a specific site, it should do so for `dist_subdir` to allow linking a
 specific distfile with a specific directory. This will help keep each
 project's distfiles in the same place and avoid duplicating the same file
 in multiple directories.

 Take for example the mysql57 port. It downloads both a mysql distfile and
 a boost distfile. We want the mysql distfile to end up in a mysql
 subdirectory of the distfiles server (and of the user's computer), and the
 boost distfile to end up in the boost subdirectory. Many other ports are
 in a similar situation.

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Ticket URL: <https://trac.macports.org/ticket/55936>
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