chore(CategoryTheory/Limits): move FormalCoproducts to new folder (leanprover-community#35016)

Author: joelriou

Mathlib.lean

@@ -2635,6 +2635,7 @@ public import Mathlib.CategoryTheory.Limits.Final.Type
 public import Mathlib.CategoryTheory.Limits.FinallySmall
 public import Mathlib.CategoryTheory.Limits.FintypeCat
 public import Mathlib.CategoryTheory.Limits.FormalCoproducts
+public import Mathlib.CategoryTheory.Limits.FormalCoproducts.Basic
 public import Mathlib.CategoryTheory.Limits.Fubini
 public import Mathlib.CategoryTheory.Limits.FullSubcategory
 public import Mathlib.CategoryTheory.Limits.FunctorCategory.Basic