fix: FlatDependencyTree type definition

[marcvincenti]

Hi, I noticed a regression in the external dependencies type introduced by #6.

Here is a minimal repro

const provide = createProvider({
  injectables: {
    foo: ({ service }: { service: number }) => service,
    service: ({ nonTypedDep }) => nonTypedDep,
  },
  api: ['foo'],
});
provide()

This behavior propagates to other external dependencies which won't be required.