runtime: extract 'runtimepath' and 'packpath' logic to its own file

No code changes, except for added ILOG for the calculated startup path

src/nvim/runtime.h

@@ -0,0 +1,18 @@
+#ifndef NVIM_RUNTIME_H
+#define NVIM_RUNTIME_H
+
+#include <stdbool.h>
+
+#include "nvim/ex_docmd.h"
+
+typedef void (*DoInRuntimepathCB)(char_u *, void *);
+
+// last argument for do_source()
+#define DOSO_NONE       0
+#define DOSO_VIMRC      1       // loading vimrc file
+
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "runtime.h.generated.h"
+#endif
+#endif  // NVIM_RUNTIME_H