How to protect source codes when attracting remote developers?

Question

Distant work

There is a task of developing software for a customer who is very sensitive to the safety of the source code, which will be transferred to him upon completion of the project.
The specifics of our development is such that we use remote programmers for individual tasks.
Our standard tools are Visual Studio + TFS. If you give access to TFS to a project, then the entire source code of the project is uploaded to the developer on the machine, I want to avoid this.
The problem is certainly not unique, and most likely someone has experience in solving it. Advise, at least in general terms, in which direction to start digging.

Answer 1

As an option: separate their tasks into separate "plugins" and provide only, so to speak, an SDK, i.e. assemblies with interfaces.

Answer 2

RDP to your virtual machines where does VisualStudio live + disabled mapping of client drives?