Implement RFC 1260 with feature_name imported_main.

compiler/rustc_error_codes/src/error_codes/E0136.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 More than one `main` function was found.
 
 Erroneous code example:
 
-```compile_fail,E0136
+```compile_fail
 fn main() {
     // ...
 }