Package name field

[bennypowers]

It occurs me that the Package entity doesn't have a name field. This may present a problem if the user only has access to the custom-elements.json, but not the package.json

for example, when pre-loading manifests into a language server, a la Matsuuu/custom-elements-language-server#20

[justinfagnani]

I think this is a good idea, especially if we see tools that combine manifests at all.

My big comment on #123 is that we need to figure out some namespacing system, since npm isn't the only package registry. There's GitHub packages, JSR, and private registries.

As a start, should we either require an npm: prefix, or assume that plain names are npm packages?

cc @mannix-lei