rustc: Disallow importing through use statements

Resolve is currently erroneously allowing imports through private `use`
statements in some circumstances, even across module boundaries. For example,
this code compiles successfully today:

    use std::c_str;
    mod test {
        use c_str::CString;
    }

This should not be allowed because it was explicitly decided that private `use`
statements are purely bringing local names into scope, they are not
participating further in name resolution.

As a consequence of this patch, this code, while valid today, is now invalid:

    mod test {
        use std::c_str;

        unsafe fn foo() {
            ::test::c_str::CString::new(0 as *u8, false);
        }
    }

While plausibly acceptable, I found it to be more consistent if private imports
were only considered candidates to resolve the first component in a path, and no
others.

Closes #12612

src/test/auxiliary/issue-12612-1.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.
+
+pub mod bar {
+    pub fn foo() {}
+}

src/test/auxiliary/issue-12612-2.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.
+
+pub fn baz() {}

src/test/compile-fail/issue-12612.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// aux-build:issue-12612-1.rs
+
+extern crate foo = "issue-12612-1";
+
+use foo::bar;
+
+mod test {
+    use bar::foo;
+    //~^ ERROR: unresolved import
+    //~^^ ERROR: failed to resolve import
+}
+
+fn main() {}
+

src/test/run-pass/issue-11881.rs

@@ -8,13 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-extern crate ser = "serialize";
+extern crate serialize;
 
-use serialize = self::ser;
- //necessary for deriving(Encodable)
-use ser::{Encodable, Encoder};
-use ser::json;
-use ser::ebml::writer;
+use serialize::{Encodable, Encoder};
+use serialize::json;
+use serialize::ebml::writer;
 use std::io::MemWriter;
 use std::str::from_utf8_owned;
 

src/test/run-pass/issue-12612.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// aux-build:issue-12612-1.rs
+// aux-build:issue-12612-2.rs
+
+extern crate foo = "issue-12612-1";
+extern crate bar = "issue-12612-2";
+
+use foo::bar;
+
+mod test {
+    use bar::baz;
+}
+
+fn main() {}