Package extensions for Julia < 1.9

Where exactly? The description in that PR is pretty self-contained, and links to CategoricalArrays forces OpenSpecFun_jll recompilation - #18 by tim.holy and Warning when conditionally loading "glue" module · Issue #65 · JuliaPackaging/Requires.jl · GitHub. Which themselves transitively link to Warning when conditionally loading "glue" module · Issue #1238 · JuliaLang/Pkg.jl · GitHub (closed as for Base), Warning when conditionally loading "glue" module · Issue #32413 · JuliaLang/julia · GitHub (no follow-up), Thoughts on how we can make Requires precompile friendly. · Issue #33 · JuliaPackaging/Requires.jl · GitHub (closed in favour of Discourse topic) and Optional dependencies / Requires.jl (contains relevant discussion but out of date). Oh, and a Slack thread where I tested the ideas in Requires.jl#65 but didn’t get much engagement after mentioning they failed.

It would be ideal if we could avoid forking this discussion onto another thread and instead continue here/consolidate it into an existing issue.