states: Improve confusing comment for DeepCopy of local values

The comment here was trying to say that we don't need to _deep-copy_
cty.Value values, but it wasn't clear about that and so it seemed weird
to have a comment saying that copying isn't needed right before a call to
a function called "Copy".

This updates the comment to be more explicit, and also takes the
opportunity to simplify to use maps.Clone instead of maps.Copy, since we
have no need for the intermediate empty map we were previously creating
prior to the Copy call.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/states/state_deepcopy.go

@@ -62,15 +62,14 @@ func (ms *Module) DeepCopy() *Module {
 	for k, v := range ms.OutputValues {
 		outputValues[k] = v.DeepCopy()
 	}
-	localValues := make(map[string]cty.Value, len(ms.LocalValues))
-	// cty.Value is immutable, so we don't need to copy these.
-	maps.Copy(localValues, ms.LocalValues)
 
 	return &Module{
 		Addr:         ms.Addr, // technically mutable, but immutable by convention
 		Resources:    resources,
 		OutputValues: outputValues,
-		LocalValues:  localValues,
+
+		// [cty.Value] is immutable, so a shallow copy is okay for local values.
+		LocalValues: maps.Clone(ms.LocalValues),
 	}
 }