proc_macro: Add a workaround for rustdoc

Author: petrochenkov

Cargo.lock

@@ -2894,6 +2894,7 @@ dependencies = [
 name = "proc_macro"
 version = "0.0.0"
 dependencies = [
+ "core",
  "std",
 ]
 

library/proc_macro/Cargo.toml

@@ -5,3 +5,7 @@ edition = "2021"
 
 [dependencies]
 std = { path = "../std" }
+# Workaround: when documenting this crate rustdoc will try to load crate named
+# `core` when resolving doc links. Without this line a different `core` will be
+# loaded from sysroot causing duplicate lang items and other similar errors.
+core = { path = "../core" }