Problem: Some use cases might lead to `vim.pack.add()` failing to
`:packadd` a plugin because of missing entry in 'packpath'. Like with
`nvim --clean` or manually setting `$XDG_DATA_HOME` during startup.
Solution: Document it. A more proactive approach can be ensuring correct
'packpath' entry, but it is currently somewhat verbose to do (due to
having to adjust for Windows using `\` in 'packpath' entries).
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