`executable-dynamic: True` should apply to `build-type: Custom` setup

I have a Haskell installation with only shared libraries, and cabal's config contains:

```
library-vanilla: False
library-profiling: False
shared: True
executable-dynamic: True
```

This fails for packages with `build-type: Custom`, because cabal-install tries to link `setup` against non-existent static libraries.

Start by tracing cabal-install's handling of packages with build-type: Custom and the setup executable when only shared libraries are configured. Reproduce the failure with the reported library-vanilla, library-profiling, shared, and executable-dynamic settings; done means setup links successfully without requiring unavailable static libraries.

Written by the indexing model from the issue text.