`lib.pipe` strictly evaluates intermediate steps using `foldl'`. As a
result, piping `(opts: opts.package.default or null)` →
`(package: (tryEval package).value)` is ineffective because
`opts.package.default` is evaluated before `tryEval` can catch exceptions.
Instead, inline `opts.package.default` directly into the `tryEval`
expression, ensuring missing package errors caught correctly.
Resolves errors when building NixOS or nix-darwin docs that include
Nixvim options.
Adds a regression test.
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