Narrow the defintion for registry field in Manifest.

[u-train]

Right now, a registry field is simply an arbitrary string. It would be nice to have a stricter definition since a registry has to point to a valid URL or directory path (the latter which is internal).

One choice is to specify that a registry has to be a valid URI. It still allows the user to specify the scheme being used (file vs http/https). We can always look at the scheme and then error if isn't valid. And, this type can be applied throughout the codebase, ensuring that we're not playing with some string, but URI specifically.

Another option is make an internal enumeration which would have File or URL. Though, I think it's just a smaller subset of a URI.