When starting a project, you should choose and clearly define a project governance model (who manages work, who integrates changes, and who is responsible for what). You should decide about the license and the copyright of the code: whether it is work for hire, whether contributions would require a copyright assignment, a contributor agreement, or a contributor license agreement, or simply a digital certificate of origin.
In centralized version control systems, often everything under the sun is put under the same project tree. With distributed version control systems such as Git, it is better to split separate projects into separate repositories.
There should be one conceptual group per repository; divide it beforehand correctly. If some part of the code is needed by multiple separate projects, consider extracting it into its own project and then incorporating it as a submodule or subtree, grouping concepts into a superproject. See Chapter...