Start replacing CStore trait methods with hooks.

This also avoids the cyclic definition issues with CrateStore being
defined after TyCtxt, but needing to be used in TyCtxt.

compiler/rustc_session/src/cstore.rs

@@ -230,12 +230,6 @@ pub trait CrateStore: std::fmt::Debug {
         index_guess: u32,
         hash: ExpnHash,
     ) -> ExpnId;
-
-    /// Imports all `SourceFile`s from the given crate into the current session.
-    /// This normally happens automatically when we decode a `Span` from
-    /// that crate's metadata - however, the incr comp cache needs
-    /// to trigger this manually when decoding a foreign `Span`
-    fn import_source_files(&self, sess: &Session, cnum: CrateNum);
 }
 
 pub type CrateStoreDyn = dyn CrateStore + sync::DynSync + sync::DynSend;