Simplify message paths

This makes it easier to open the messages file while developing on features.

The commit was the result of automatted changes:

for p in compiler/rustc_*; do mv $p/locales/en-US.ftl $p/messages.ftl; rmdir $p/locales; done

for p in compiler/rustc_*; do sed -i "s#\.\./locales/en-US.ftl#../messages.ftl#" $p/src/lib.rs; done

compiler/rustc_session/src/lib.rs

@@ -42,7 +42,7 @@ pub mod output;
 
 pub use getopts;
 
-fluent_messages! { "../locales/en-US.ftl" }
+fluent_messages! { "../messages.ftl" }
 
 /// Requirements for a `StableHashingContext` to be used in this crate.
 /// This is a hack to allow using the `HashStable_Generic` derive macro