In managing my development projects, historically I would break my solutions up unto modular projects, each which could be used by multiple solutions. This allowed the modular project 'library' to be developed in parallel with the main program.
In practice this was difficult as adding existing projects to many solutions meant coming back to any solution would almost always require updating the code as the 'library' would likely have changed in the meantime.
Using git submodules was a help, since each solution could have its own codebase at its own point in the development cycle. The ultimate usage then was to have a separate branch of a submodule for every solution that consumed it, where the updates could more gracefully be merged into other programs.
Nuget packages for submodules seems to be a great step forward in handling submodule versioning, but it takes away the ability to directly develop the submodule in conjunction with the consuming solution, which is slowing development significantly - each update to the submodule requires a new build, repackage and reinstall into the consumer.
Is there a better solution to using the benefits of nuget packaging while being able to develop solutions and submodules in conjunction with one another, something like having the code for the sumbodule in the solution, but keeping the nuget reference?