New Guide: crates and modules

Also fixes a couple of filesystem references that I forgot to change. :sweat_smile: 
It's actually required.

```
$ tree
.
|-- bar.rs
|-- foo.rs
`-- main.rs
$ cat main.rs
mod foo;

fn main() {}
$ cat foo.rs
mod bar;
$ cat bar.rs
// ...
$ rustc main.rs
foo.rs:1:5: 1:8 error: cannot declare a new module at this location
foo.rs:1 mod bar;
             ^~~
foo.rs:1:5: 1:8 note: maybe move this module `foo` to its own directory via `foo/mod.rs`
foo.rs:1 mod bar;
             ^~~
foo.rs:1:5: 1:8 note: ... or maybe `use` the module `bar` instead of possibly redeclaring it
foo.rs:1 mod bar;
             ^~~
error: aborting due to previous error
$
$ mkdir foo && mv foo.rs foo/mod.rs && mv bar.rs -t foo
$ tree
.
|-- foo
|   |-- bar.rs
|   `-- mod.rs
`-- main.rs

$ rustc main.rs
$
```