update zlib.cabal location

The upstream zlib package was recently reorganized in a way that
caused our git fetch test to fail. Update it to match.

Author: geekosaur

cabal-install/tests/UnitTests/Distribution/Client/Get.hs

@@ -216,7 +216,7 @@ testNetworkGitClone =
       Nothing
       []
       [(mkpkgid "zlib1", [repo1])]
-    assertFileContains (tmpdir </> "zlib1/zlib.cabal") ["name:", "zlib"]
+    assertFileContains (tmpdir </> "zlib1/zlib/zlib.cabal") ["name:", "zlib"]
 
     let repo2 =
           (emptySourceRepo RepoHead)