unhide nuget packages/ for VSCode (#15579)

Don't exclude nuget `packages/` in vscode. Excluding via `file.exclude`
also excludes them from c++ language extension's `includePath` and
generates missing include files and header errors.

We might still like to exclude them from full text search, so we do it
using `search.exclude`.

Closes #15578

.vscode/settings.json

@@ -103,7 +103,9 @@
     "files.exclude": {
         "**/bin/**": true,
         "**/obj/**": true,
-        "**/packages/**": true,
         "**/Generated Files/**": true
+    },
+    "search.exclude": {
+        "**/packages/**": true
     }
 }