CPP inside import hiding list is thrown away

See https://github.com/ekmett/trifecta/pull/73/commits/90d3ea3d568b41806b36910d7a3f7ba19b48573c#diff-5ce974d71a85ffa9de2bd3160230ad6aL61:

Before:
```haskell
import Control.Lens
hiding ((<|),(|>)
#if MIN_VERSION_lens(4,13,0) && __GLASGOW_HASKELL__ >= 710
,(:<)
#endif
)
```

After applying stylish-haskell:

```haskell
import Control.Lens hiding ((:<), (<|), (|>))
```

Although I'm able to work around this problem, I'd rather have `stylish-haskell` leave it as-is, or refuse to format this file, than throwing away CPPs and thus inadvertedly breaking the build.

Start with the linked stylish-haskell diff and reproduce the formatting of the CPP-guarded import example from the issue. Trace how the import's hiding list is processed, then verify that CPP directives are preserved or the file is refused rather than silently changed.

Written by the indexing model from the issue text.