Make generated code classes file private

[captainsafia]

Follow-up to #45924.

This updates the code generated by the RequestDelegateGenerator so that all classes emitted are file private except for:

• The SourceKey class which is currently used in tests to ensure that endpoints were successfully generated at compile-time.
• The GenerateRouteBuilderEndpoints which needs to be accessible in order to have the endpoints it contains participate in overload resolution.

[halter73]

• The GenerateRouteBuilderEndpoints which needs to be accessible in order to have the endpoints it contains participate in overload resolution.

Can we add the source generated IEndpointRouteBuilder extension methods to the same "file" as the caller so we can make those file-local too?

[captainsafia]

Can we add the source generated IEndpointRouteBuilder extension methods to the same "file" as the caller so we can make those file-local too?

The caller in this case is the user's code so it isn't possible. For example, if the user's MapAction call is this:

app.MapGet("/hello", (int id) => $"Id: {id}");

Then the generated endpoint that needs to be accessible for resolution is:

MapGet(string pattern, Func<int, string> handler)