Add support for sorting dart files in `packages` folder

[bhav-khurana]

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

[vkammerer]

This is something that could be handled by the next maintainer once #84 is resolved