src: Fix 'contains_key' clippy lints

[travier]

warning: unnecessary use of `get(k).is_some()`
   --> src/filetree.rs:187:34
    |
187 |                 if self.children.get(k).is_some() {
    |                                  ^^^^^^^^^^^^^^^^ help: replace it with: `contains_key(k)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check
    = note: `#[warn(clippy::unnecessary_get_then_check)]` on by default