Fix package code for new NuGet.

The new NuGet uses tolower paths, and we need to react to it.

Port https://github.com/dotnet/cli/pull/2722
Port https://github.com/dotnet/cli/pull/3554

TestAssets/LockFiles/ExportFiles/valid/project.lock.json

@@ -57,5 +57,9 @@
       "ClassLibrary2",
       "ClassLibrary3"
     ]
+  },
+  "packageFolders": {
+    "/foo/packages": {},
+    "/foo/packages2": {}
   }
 }