Do less file system crawling when we have the Dart dependency set (#9424)

- [x] Skip scanning the file system if we already have the Dart dependency set.

Fixes #9376

```
Performing hot reload...
Reloaded 1 of 418 libraries in 888ms.

Performing hot reload...
Reloaded 1 of 418 libraries in 871ms.

** UNTAR dragontail under project root **
Performing hot reload...
Reloaded 0 of 418 libraries in 443ms.

** UNTAR dragontail under lib/ **
Performing hot reload...
Reloaded 0 of 418 libraries in 385ms.
```