Module import is considered unused if it used only for an extension method used in a CE

**Repro steps**
Consider the following code:

```fsharp
open System.Threading.Tasks

module Builder =

type AsyncBuilder with
// allows to bind to the result of Tasks
member _.Bind (task : Task<'a>, cont : 'a -> Async<'b>) =
async.Bind (Async.AwaitTask task, cont)

open Builder

let task = Task.FromResult 1
let x = async {
let! i = task
return i + 1
}
```

Here the `Builder` module contains an extension method for an `AsyncBuilder` that enables the former to bind to the results of usual .Net tasks.

**Expected behavior**

No compiler warnings or errors.

**Actual behavior**

F# compiler considers this import to be not required:
Warning "IDE0005: Open declaration can be removed".


But it is required as without it the compiler generates an error:
```
X.fs(16, 14): [FS0001] This expression was expected to have type
'Async<'a>'
but here has type
'Task'
```

**Related information**

* Operating system — Windows 10
* .NET Framework 4.7.2
* Visual Studio 16.10.0

Inizia riproducendo il codice F# mostrato nell’issue e confronta l’avviso IDE0005 con l’errore del compilatore dopo aver rimosso `open Builder`. Traccia il modo in cui l’uso dei metodi di estensione all’interno della computation expression viene considerato dall’analisi degli open inutilizzati. Il lavoro è terminato quando l’import richiesto non viene segnalato come rimovibile e la computation expression continua a essere verificata correttamente dal sistema dei tipi.

Scritto dal modello di indicizzazione a partire dal testo della issue.