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/test/compile-fail/circular_modules_main.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-
+#[path = "circular_modules_hello.rs"]
 mod circular_modules_hello; //~ERROR: circular modules
 
 pub fn hi_str() -> StrBuf {

src/test/compile-fail/mod_file_not_owning.rs

@@ -0,0 +1,15 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern: cannot declare a new module at this location
+
+mod mod_file_not_owning_aux1;
+
+fn main() {}

src/test/compile-fail/mod_file_not_owning_aux1.rs

@@ -0,0 +1,13 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-test this is not a test
+
+mod mod_file_not_owning_aux2;

src/test/compile-fail/mod_file_not_owning_aux2.rs

@@ -0,0 +1,11 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-test this is not a test

src/test/run-pass/mod_dir_simple/load_another_mod.rs

@@ -8,4 +8,5 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[path = "test.rs"]
 pub mod test;