Scanning of ELF files for missing shared libraries a la `revdep-rebuild`

This could be an optional feature to supplement the current querying of `ghc-pkg check`. It can limit the scan to files that are owned by gentoo-haskell packages, as these packages register themselves in `$(ghc --print-libdir)/gentoo/`.

---

something like

```bash
haskell-updater --elf-scan
```