Add support for sorting dart files in `packages` folder

**Is your feature request related to a problem? Please describe.**
Yes, some of the projects use a structure where only the `main.dart` file is present in the `lib` folder and the features of the app are present as packages in the `packages` folder.

**Describe the solution you'd like**
The import_sorter package should also run on the dart files present in the `packages` folder if it exists.

**Describe alternatives you've considered**
An alternative solution to this will be to write a script that goes through each folder in `packages` and run `dart run import_sorter:main file_name`. But it would be better if it's included in the package itself.

**Additional context**
NA