Unused opens: extension member in namespace is ignored in analysis

In the following code `P1` extension is declared in a namespace, it's not allowed. The symbol is successfully resolved in another namespace (it's good for the tooling), but is not accounted in unused opens analysis.
```fsharp
namespace Ns1

type System.String with
member this.P1 = 1

namespace Ns2

open Ns1

module Module =
"".P1
```

`open Ns1` is considered unused:

Works as expected for extension in a module: