Cabal exposed-modules completion ignores stanza type

## Problem
CabalCompletionContributor suggests module names for exposed-modules without considering the current stanza type, and currently behaves as if only project library module names are valid.

## Why it matters
This can produce incorrect completion suggestions in non-library stanzas and makes Cabal editing less reliable.

## Affected code locations
- https://github.com/ForNeVeR/haskeletor/blob/d71af7e001f34c8115596d70af0994889b80017d/src/main/scala/me/fornever/haskeletor/cabal/completion/CabalCompletionContributor.scala#L58