Everything in packages/
is (usually) a symlink to a possibly shared copy of a package, so if you edited a file in packages/
you'd be editing it for all your projects, which might be very not what you want.
If you'd like to edit multiple packages together, the best way to do it is to specify a dependency override that uses a path source, like so:
name: my_package
dependency_overrides:
my_other_package:
path: /Users/me/dart/my_other_package
This way any other dependency on that package will also load it from the specified path and pub won't complain that you have different sources for the same package. Then you can open both projects separately in the editor and the my_package
will see the changes in my_other_package
as you edit.