Problem: Some newly introduced files used includes relative to the
current file, both of the form `include "../XXX.h"` and
`include "XXX.h"`.
Preferred form is relative to include root (src/ in our case).
Solution: Change includes to preferred form.
Note: This is also done to ease next commit (prepend 'nvim/ to all
project-local includes).
- Leave src as include dir (for includes to recognize 'nvim/' prefix).
- Change subdirectory from src to src/nvim.
- Fix msgpack generation.
- Fix some other paths to new locations.
Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.
