Move LSP bits from flycheck to rust-analyzer

There should be only one place that knows about LSP, and that place is
right before we spit JSON on stdout.

crates/rust-analyzer/src/diagnostics.rs

@@ -1,4 +1,5 @@
 //! Book keeping for keeping diagnostics easily in sync with the client.
+pub(crate) mod to_proto;
 
 use std::{collections::HashMap, sync::Arc};