Generated interfaces should handle composite schemas between files

[jdenue]

Sometimes a schema will include a type defined in a separate file. For example:

Type.avsc:

{
  "type" : "record",
  "name" : "Type",
  "namespace" : "com.example",
  "fields" : [
   {
      "name" : "id",
      "type" : "long",
      "default" : null
    },
    {
      "name" : "subType",
      "type" : "com.example.SubType",
      "default" : null
    }
} 

SubType.avsc:

{
  "type" : "record",
  "name" : "SubType",
  "namespace" : "com.example",
  "fields" : [
   {
      "name" : "id",
      "type" : "long",
      "default" : null
    },
    {
      "name" : "value",
      "type" : "string",
      "default" : null
    }
}

When we generate interfaces from a directory, the tool should be smart enough to scan the directory for dependencies like these, and include the appropriate typings and schemas for serialization/deserialization.