Fix #6209 - introduce standalone dir crate

* created the dir crate in util
* moved code from ethstore/src/dir/paths.rs to dir crate
* rename dir module in ethstore to accounts_dir to distinguish it
  from the dir crate
* changes after @tomusdrw on #6952

ethstore/tests/util/transient_dir.rs

@@ -17,7 +17,7 @@
 use std::path::PathBuf;
 use std::{env, fs};
 use rand::{Rng, OsRng};
-use ethstore::dir::{KeyDirectory, RootDiskDirectory};
+use ethstore::accounts_dir::{KeyDirectory, RootDiskDirectory};
 use ethstore::{Error, SafeAccount};
 
 pub fn random_dir() -> PathBuf {