As mentioned in the previous commit, we don't have a use case for
this in flake-parts yet, as far as I'm aware. It can be exposed
when we do have a concrete use case where it is needed.
The difference is that moduleLocation is "guaranteed" reliable data,
whereas errorLocation is the best choice for error messages in the core.
moduleLocation is suitable for the module key attribute.
errorLocation is best for the *ROOT* module _file attribute.
Initially I applied errorLocation in too many places. It is only needed
when the flake output attribute names are strict in it.
To avoid confusion, I'm not exposing errorLocation to the modules, until
we have a concrete use case for it.
Not how I would recommend to do it, because I'm forcefully trying
to avoid adding dependencies to flake-parts.
Other projects should not follow this restriction.
