Now that core has access to the provider configuration schema, our input
logic can be implemented entirely within Context.Input, removing the need
to execute a full graph walk to gather input.
This commit replaces the graph walk call with instead just visiting the
provider configurations (explicit and implied) in the root module, using
the schema to prompt.
The code to manage the input graph walk is not yet removed by this commit,
and will be cleaned up in a subsequent commit once we've made sure there
aren't any other callers/tests depending on parts of it.
