proc_macro: Add a workaround for rustdoc

Author: petrochenkov

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" }