Reference .wit files from outside .csproj directory

It would be nice if it were possible to reference .wit files from outside the .csproj directory, such as:
```xml

```

However this doesn't currently seem to generate the bindings.

This would be useful to share .wit files between importing and exporting projects, rather than having duplicate files that need to be kept in sync.

Use the sample .csproj ItemGroup with ..\calculator.wit as the first reproduction, then trace how the external .wit file is handled during binding generation. Done means a .wit file outside the .csproj directory generates bindings and can be shared by importing and exporting projects without duplication.

索引モデルが issue の本文から書いたものです。