Allow inner attributes on external module declarations.

[brson]

Directory modules currently require #[path], but the current formatting options when a bunch of these declarations appear together are not good. This is very unfortunate since these subdirectory declarations are often one of the first bits of code in a crate.

This variation is hard to read because the alternation of syntaxes between lines

#[path = "foo/foo.rs"]
mod foo;
#[path = "barkeeper/barkeeper.rs"]
mod barkeepr;
#[path = "baz/baz.rs"]
mod baz;

And this one because the mods don't line up:

#[path = "foo/foo.rs"] mod foo;
#[path = "barkeeper/barkeeper.rs"] mod barkeeper;
#[path = "baz/baz.rs"] mod baz;

This one reads a bit better but isn't possible:

mod foo #[path = "foo/foo.rs"];
mod barkeeper #[path = "barkeeper/barkeeper.rs"];
mod baz #[path = "baz/baz.rs"];

Of course this may just indicate that the way we are dealing with subdirectories is bad.

[catamorphism]

Nominating for milestone 1, well-defined.

[brson]

Still not convinced this is a good idea fwiw.

[graydon]

closing as WONTFIX, #4116 should solve most cases.