* Used a simple `String.slice` to create relative paths instead of `path.relative_path_from(root)`. Since we already know that all files must exist under `root` (because we globbed `absolute_paths` from `root`), we can just remove the first `root.to_s.length + 1` characters from an `absolute_path` to get a `relative_path`.
* Partition `dirs` and `files` first since we already get an array of absolute paths back from `Pathname.glob`, removing unnecessary string concat: `File.directory?(root + path)`
* Used `lazy` arrays to reduce array iteration.

I [profiled my code before the change](https://github.com/CocoaPods/CocoaPods/files/477973/call-stack.zip).
I [profiled my new code again](https://github.com/CocoaPods/CocoaPods/files/478512/call-stack2.html.zip), and `read_file_system` practically disappears.

previous:

```
real     0m59.095s
user     0m46.133s
sys      0m2.360s
```

now:

```
real     0m23.712s
user     0m14.492s
sys      0m2.030s
```