Duplicate item name when oneof name matches nested item name

[davidzeng0]

The following code generates

message Outer {
	message Inner {}

	oneof inner {
		Inner inner_one = 1;
		Inner inner_two = 2;
	}
}

// This file is @generated by prost-build.
#[derive(Clone, Copy, PartialEq, ::prost::Message)]
pub struct Outer {
    #[prost(oneof = "outer::Inner", tags = "1, 2")]
    pub inner: ::core::option::Option<outer::Inner>,
}
/// Nested message and enum types in `Outer`.
pub mod outer {
    #[derive(Clone, Copy, PartialEq, ::prost::Message)]
    pub struct Inner {} // <-- CONFLICT!

    #[derive(Clone, Copy, PartialEq, ::prost::Oneof)]
    pub enum Inner { // <-- CONFLICT!
        #[prost(message, tag = "1")]
        InnerOne(Inner),
        #[prost(message, tag = "2")]
        InnerTwo(Inner),
    }
}