Support `<name>/mod.rs` modules in the directory

[acheronfail]

This works great for all files in a directory, but I have a directory structure like this:

• src
  • items <-- automod used to source all files in this dir
    • a.rs
    • b.rs
    • c <-- skipped because it's a directory
      • mod.rs
    • d.rs
    • ...

I'd love for this to work for modules which are <name>/mod.rs, too.

My use-case is that I split out the c mod into separate files because it was getting to large (and also depended on some codegen I wanted to keep separate), etc.