Right now, config.Key is a type alias for tokens.ModuleMember. I did a
pass over the codebase such that we use config.Key everywhere it
looked like the value did not leak to some external process (e.g a
resource provider or a langhost).
Doing this makes it a little clearer (hopefully) where code is
depending on a module member structure (e.g. <package>:config:<value>)
instead of just an opaque type.
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