Add a command to help people figure out why schemes aren't built

We should add a command to help people figure out why schemes aren't built.

Something like this:

```
$ carthage diagnose --platform ios
* path/to/Bar.xcodeproj
* Foo.xcscheme
✓ Framework
X Dynamic Framework
* Flab.xcscheme
✓ Framework
✓ Dynamic Framework
✓ iOS
* Baz.xcodeproj
* No shared schemes
```

Start by locating Carthage's command entry point and the existing scheme-discovery logic used for --platform ios. Define the diagnose output around each Xcode project, shared scheme, and platform/framework checks shown in the example. Done means the command explains why schemes are excluded and reports projects with no shared schemes.

Written by the indexing model from the issue text.