internal/rosa: use string pair for files

This is a much cleaner representation than the separator syntax.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/azalea/evaluate.go

@@ -91,7 +91,7 @@ type TypeError struct {
 }
 
 func (e TypeError) Error() string {
-	return "expected " + e.Asserted.String() + ", got " + e.Concrete.String()
+	return fmt.Sprintf("expected %s, got %s", e.Asserted, e.Concrete)
 }
 
 func (e TypeError) Is(err error) bool {