Introduce nvim namespace: Fix relative includes.

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).

src/nvim/api/buffer.c

@@ -6,8 +6,8 @@
 #include "api/buffer.h"
 #include "api/helpers.h"
 #include "api/defs.h"
-#include "../vim.h"
-#include "../buffer.h"
+#include "vim.h"
+#include "buffer.h"
 #include "memline.h"
 #include "memory.h"
 #include "misc1.h"
@@ -16,7 +16,7 @@
 #include "mark.h"
 #include "fileio.h"
 #include "move.h"
-#include "../window.h"
+#include "window.h"
 #include "undo.h"
 
 // Find a window that contains "buf" and switch to it.

src/nvim/api/helpers.c

@@ -4,9 +4,9 @@
 
 #include "api/helpers.h"
 #include "api/defs.h"
-#include "../vim.h"
-#include "../buffer.h"
-#include "../window.h"
+#include "vim.h"
+#include "buffer.h"
+#include "window.h"
 #include "memory.h"
 #include "eval.h"
 #include "option.h"

src/nvim/api/helpers.h

@@ -4,7 +4,7 @@
 #include <stdbool.h>
 
 #include "api/defs.h"
-#include "../vim.h"
+#include "vim.h"
 
 #define set_api_error(message, err)                \
   do {                                             \

src/nvim/api/vim.c

@@ -7,9 +7,9 @@
 #include "api/helpers.h"
 #include "api/defs.h"
 #include "api/buffer.h"
-#include "../vim.h"
-#include "../buffer.h"
-#include "../window.h"
+#include "vim.h"
+#include "buffer.h"
+#include "window.h"
 #include "types.h"
 #include "ascii.h"
 #include "ex_docmd.h"

src/nvim/api/window.c

@@ -5,8 +5,8 @@
 #include "api/window.h"
 #include "api/defs.h"
 #include "api/helpers.h"
-#include "../vim.h"
-#include "../window.h"
+#include "vim.h"
+#include "window.h"
 #include "screen.h"
 #include "misc2.h"
 

src/nvim/os/msgpack_rpc.c

@@ -1,6 +1,6 @@
 #include <msgpack.h>
 
-#include "msgpack_rpc.h"
+#include "os/msgpack_rpc.h"
 #include "vim.h"
 #include "memory.h"