When I was working on my Goodreads Companion app, due to the way GR had implemented authorisation on their site I had to make changes to the oauth1 package on pub.dev.
I found this article explaining how to do it:
In summary:
Fork the repo of the package
Clone the repo
Make your changes and push
Add something like the following to your pubspec
Last updated 5 years ago
dependencies: flutter: sdk: flutter [package-name]: git: url: https://github.com/[git-username]/[git-repository-name]