lint(dirs): collapse unnecessarily nested ifs

Ref:
```
Checking statix v0.5.8 (/build/source/bin)
error: this `if` statement can be collapsed
--> bin/src/dirs.rs:50:17
|
50 | / if dir.is_dir() {
51 | | if let Match::None | Match::Whitelist(_) = self.ignore.matched(&dir, true) {
52 | | let mut found = false;
53 | | for entry in fs::read_dir(&dir).ok()? {
... |
70 | | }
| |_________________^
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
= note: `-D clippy::collapsible-if` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::collapsible_if)]`
help: collapse nested if block
|
50 ~ if dir.is_dir()
51 ~ && let Match::None | Match::Whitelist(_) = self.ignore.matched(&dir, true) {
52 | let mut found = false;
...
68 | }
69 ~ }
|

error: could not compile `statix` (lib) due to 1 previous error
```