syntax: Tighten search paths for inner modules

This is an implementation of RFC 16. A module can now only be loaded if the
module declaring `mod name;` "owns" the current directory. A module is
considered as owning its directory if it meets one of the following criteria:

* It is the top-level crate file
* It is a `mod.rs` file
* It was loaded via `#[path]`
* It was loaded via `include!`
* The module was declared via an inline `mod foo { ... }` statement

For example, this directory structure is now invalid

    // lib.rs
    mod foo;

    // foo.rs
    mod bar;

    // bar.rs;
    fn bar() {}

With this change `foo.rs` must be renamed to `foo/mod.rs`, and `bar.rs` must be
renamed to `foo/bar.rs`. This makes it clear that `bar` is a submodule of `foo`,
and can only be accessed through `foo`.

RFC: 0016-module-file-system-hierarchy
Closes #14180

[breaking-change]

src/libsyntax/ext/source_util.rs

@@ -95,6 +95,8 @@ pub fn expand_include(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                                         &res_rel_file(cx,
                                                       sp,
                                                       &Path::new(file)),
+                                        true,
+                                        None,
                                         sp);
     base::MacExpr::new(p.parse_expr())
 }