cabal sdist target selection is broken

```
% cabal sdist splitmix
Error: [Cabal-7151]
The component library cannot be packaged for distribution on its own. Only entire packages may be packaged for distribution.
```

`splitmix` is the name of package. `cabal-install` is hallucinating.

Start by reproducing `cabal sdist splitmix` and tracing the `sdist` target-selection entry point that produces Cabal error 7151. Confirm why the package name is treated as a component library, then verify that the command packages the whole `splitmix` package without reporting the component error.

Written by the indexing model from the issue text.