Modern CMake is usually target-driven in that a target is first defined
and then the likes of `target_sources`, `target_include_directories`
etc. are used to further populate the target. We still use old-style
CMake, where we first set up a set of variables and then populate the
target in a single call.

Let's migrate to modern CMake usage by starting to populate the sources
of our git2internal target piece-by-piece. While this is a small step,
it allows us to convert to target-based build instructions
piece-by-piece.