terraform: BuiltinEvalContext function eval in partial-expanded module

[apparentlymart]

https://github.com/hashicorp/terraform/pull/34571 and https://github.com/hashicorp/terraform/pull/34614 previously changed the EvalContext interface to support evaluation in both fully-expanded and partial-expanded modules, but the provider-contributed functions feature landed semi-concurrently with it and so inadvertently introduced a codepath that only worked in the fully-expanded case.

This will now handle both situations, since all we really need is the addrs.Module, which we can obtain in both modes.