Share plug libraries with git submodules

Keep shared code such as a plug library in its own repository: deployment builds now fetch the procedure's git submodules.

• Added git submodules to deployment builds, nested ones included, with HTTPS, relative or SSH URLs. SSH URLs need no key.
• Added access to private submodule repositories through your git integration. On GitHub, add the repository to the TofuPilot GitHub App. Public submodules work from any host.
• Added clear build errors when a submodule cannot be fetched, naming the submodule and what to fix.