Access modifier `type private X with ...` type extensions is ignored

A private type extension `type private X with` inside a module will be in scope after the module is opened.

#### Repro steps

```fsharp
module Test

module Inner =
type private System.String with
member _this.ExtensionOnString () = ()

"".ExtensionOnString () // Works here.

open Inner
"".ExtensionOnString () // Works too. [A]
```

#### Expected behavior

The line marked [A] should fail to compile, since the type extension is private to the `Inner` module.

#### Actual behavior

The code works.

#### Known workarounds
Placing all extensions in another private module works.

```fsharp
module Test

module Inner =
module private Extensions =
type System.String with
member _this.ExtensionOnString () = ()

open Extensions
"".ExtensionOnString () // Works here.

open Inner
// "".ExtensionOnString () but no longer here.
```

#### Related information

The same on .NET and Mono.