VERSIONDEFS: consider extending SOURCE parameter to designate versions

[masinter]

Discussed at the May 9 2026 meeting in the context of PR #2530.

There was sympathy for the idea of using different kinds of parameters to the SOURCE parameter of GETDEF (and related filepkg operations) as a way of designating which versions were wanted — e.g., by Medley version number, git commit, or date. This would be a more principled approach than adding new functions.

However, doing so might require rearchitecting the VERSIONDEFS lispusers package. This issue tracks that future work.

Related ideas raised in discussion:

• Version numbers could reference versions of the actual definition (not just the file), so -3 would be the third-back version of a specific function even if it spanned multiple commits
• Specifying versions by date would also be useful
• GITFNS would need a primitive for fetching and interpreting history