GHCup repositories

Right now we have a single huge .yaml that covers all tools.

It's at least worth discussing whether we can or should shell out some of the work to upstream devs (e.g. HLS @jneira ). So GHCup would fetch a yaml from the HLS developers and add that metadata to the core metadata.

There are some trade-offs to consider:

1. security (multiple sources... we don't do gpg verification yet)
2. performance (every web request adds to the overhead and failure modes... we already do caching, but it will be noticeable and may need better design)
3. ergonomics (will it make things more complicated?)