Add changelog for haskell#7966 and haskell#7776

Author: jneira

changelog.d/pr-7966

@@ -0,0 +1,9 @@
+synopsis: Fix the duplicate modules check
+packages: Cabal
+prs: #7966
+issues: #7776
+
+description: {
+- Now if a package may have duplicate modules (subject to conditionals), cabal warns on this.
+- But if a package necessarily has duplicate modules (because they are not guarded by conditionals), cabal will error.
+}