Add visibility declarations to internal libraries.

This commit makes the visibility of each library — whether it is public
or private — explicit. It shouldn't change the actual visibility of any
internal library.

If an internal library lacks an explicit visibility declaration, cabal
treats it as private by default.

See:
https://cabal.readthedocs.io/en/3.4/cabal-package.html#pkg-field-library-visibility

Author: jonathanknowles

monoidmap.cabal

@@ -119,6 +119,8 @@ library monoidmap-examples
       , extensions
     build-depends:
       , monoidmap
+    visibility:
+        private
     hs-source-dirs:
         components/monoidmap-examples
     exposed-modules:
@@ -143,6 +145,8 @@ library monoidmap-internal
       , dependency-monoid-subclasses
       , dependency-nothunks
       , extensions
+    visibility:
+        private
     hs-source-dirs:
         components/monoidmap-internal
     exposed-modules: