proc_macro: move the rustc frontend to syntax_ext::proc_macro_impl.

Author: eddyb

src/Cargo.lock

@@ -7,8 +7,3 @@ version = "0.0.0"
 path = "lib.rs"
 crate-type = ["dylib"]
 
-[dependencies]
-syntax = { path = "../libsyntax" }
-syntax_pos = { path = "../libsyntax_pos" }
-rustc_errors = { path = "../librustc_errors" }
-rustc_data_structures = { path = "../librustc_data_structures" }

src/libproc_macro/Cargo.toml

@@ -48,10 +48,6 @@ pub mod bridge;
 
 use bridge::{Frontend, FrontendInterface};
 
-#[unstable(feature = "proc_macro_internals", issue = "27812")]
-#[doc(hidden)]
-pub mod rustc;
-
 mod diagnostic;
 
 #[unstable(feature = "proc_macro", issue = "38356")]