Reintroduce directory modules

[brson]

As part of #2176 I removed directory modules from the language. As a result, crates make heavy use of #[path] to create directory structure.

We probably do want to have some directory convention that rustc understands.

Here's one idea: the mod foo; statement continues to be the only way to load external modules. The parser looks for foo.rs and foo/mod.rs. If either exists then it loads it, otherwise error.

This assumes that we like the mod.rs convention. I haven't decided yet.

[nikomatsakis]

I believe we agreed in a meeting to stick with the mod.rs convention
(which is also compatible with graydon's proposed rustpkg conventions)

[nikomatsakis]

Not critical for 0.6; removing milestone

[brson]

Additional mailing list discussion https://mail.mozilla.org/pipermail/rust-dev/2013-February/003204.html

[brson]

I would like to nominate this for a milestone. It is important for usability.

Let's also remove all the redundant uses of #[path].

[brson]

Done