cmake: move library build instructions into subdirectory

To fix leaking build instructions into different targets and to make
the build instructions easier to handle, create a new CMakeLists.txt
file containing build instructions for the libgit2 target.

By now, the split is rather easy to achieve. Due to the preparatory
steps, we can now simply move over all related build instructions, only
needing to remove the "src/" prefix from some files.