terraform: BuiltinEvalContext function eval in partial-expanded module

An earlier commit changed the EvalContext model 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.

internal/terraform/eval_context_builtin.go

@@ -489,7 +489,7 @@ func (ctx *BuiltinEvalContext) evaluationExternalFunctions() lang.ExternalFuncs
 	// by the module author.
 	ret := lang.ExternalFuncs{}
 
-	cfg := ctx.Evaluator.Config.DescendentForInstance(ctx.Path())
+	cfg := ctx.Evaluator.Config.Descendent(ctx.scope.evalContextScopeModule())
 	if cfg == nil {
 		// It's weird to not have a configuration by this point, but we'll
 		// tolerate it for robustness and just return no functions at all.