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/parse/mod.rs

@@ -184,8 +184,13 @@ pub fn new_parser_from_file<'a>(sess: &'a ParseSess,
 pub fn new_sub_parser_from_file<'a>(sess: &'a ParseSess,
                                     cfg: ast::CrateConfig,
                                     path: &Path,
+                                    owns_directory: bool,
+                                    module_name: Option<StrBuf>,
                                     sp: Span) -> Parser<'a> {
-    filemap_to_parser(sess, file_to_filemap(sess, path, Some(sp)), cfg)
+    let mut p = filemap_to_parser(sess, file_to_filemap(sess, path, Some(sp)), cfg);
+    p.owns_directory = owns_directory;
+    p.root_module_name = module_name;
+    p
 }
 
 /// Given a filemap and config, return a parser