hackage-db: `signatures` field in `MetaData` has the wrong type

It's not a list of strings, it's a list of structured objects. They look like this:

```
{
  "keyid":"90a6b6d70ee8a2467cdacfc42a6ae2b22d4676b240210881b32a9eb55ecc8e58",
  "method":"ed25519",
  "sig":"K4nOQyuRq0JmbsdevazQWHzmQ0YtxCOXaFqi3kdnHipZXIy/G15RNPGJhhK0iMfPaW57PzrTj6foZJAZNxMYAw=="
}
```

I'll make a patch.