tree-wide: Introduce sd-forward.h and shared-forward.h headers

Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.

The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.

Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.

src/basic/sparse-endian.h

@@ -25,7 +25,7 @@
 #include <byteswap.h>
 #include <endian.h>
 
-#include "forward.h"
+#include "basic-forward.h"
 
 #ifdef __CHECKER__
 #define __sd_bitwise __attribute__((__bitwise__))