#[proc_macro_hack] reexport cannot handle leading :: in path

[danielhenrymantilla]

As we can see from the code, when using the #[proc_macro_hack] attribute to "decorate" a reexport (i.e., in the "declaration crate"), it expects to start parsing

use $implementation_crate_name:ident ::

However, given that we are reexporting from a crate, a way to do this in the 2018 edition (the cleanest way imho, since it avoids any potential path ambiguity), would be to use a leading pair of colons:

#[proc_macro_hack]
use ::implementation_crate_name::proc_macro_name;

That's why it would be great if proc_macro_hack in reexport position was changed so that it expects to start parsing

use $(::)? $implementation_crate_name:ident ::

[dtolnay]

Thanks, I would accept a PR to fix this.