Macro path resolution ignores preludes

[petrochenkov]

All kinds of them - extern prelude, standard library prelude, language prelude, except for the macro prelude of course (which contains of builtin macros and macros from #[macro_use] extern crate).

#![feature(extern_prelude, use_extern_macros)]
#![allow(unused)]

mod m {
    fn f() {
        std::mem::drop(0); // OK, extern prelude
        Result::Ok::<u8, u8>(0); // OK, standard library prelude
        u8::clone(&0); // OK, language prelude
        
        std::panic!(); // ERROR failed to resolve. Use of undeclared type or module `std`
        Result::Ok!(); //  ERROR failed to resolve. Use of undeclared type or module `Result`
        u8::clone!(); // ERROR failed to resolve. Use of undeclared type or module `u8`
    }
}

fn main() {}

[petrochenkov]

The fix is potentially a minor breaking change due to certain kinds of shadowing being prohibited in macro paths:

#![feature(decl_macro)]
#![allow(unused)]

mod m {
    fn f() {
        macro_rules! mac { () => {
            mod std {
                pub macro panic() {}
            }
        }}
        
        std::panic!(); // should be an ERROR: ambiguous prelude vs macro-expanded module
                            // not an error now due to prelude being ignored
        
        mac!();
    }
}

fn main() {}

I think we can leave this to post-1.2 though.