This makes definitions like
home.activation.foo = mkIf false "bar"
work, where previously they would complain about
`home.activation.foobar.data` being used but not defined.
The crucial part is that we don't call `convertAllToDags` in
`dagOf.merge`, because we need to process `mkIf`/`mkMerge` properties
first. So we let `attrEquivalent.merge` do its job normally, but give
it a type `dagEntryOf` that does the conversion.
Ideally this shouldn't require so much boilerplate; I'd like to
implement something like
types.changeInto dagContentType elemType dagEntryAnywhere
in Nixpkgs.
(cherry picked from commit 8db712a6a2)
Given an inner type, the former function generates a type that expect
DAG option values. The latter function is only present to temporarily
allow the `programs.ssh.matchBlocks` to keep accepting list values.
