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/string-util.h

@@ -4,7 +4,7 @@
 #include <string.h>
 
 #include "alloc-util.h"
-#include "forward.h"
+#include "basic-forward.h"
 #include "string-util-fundamental.h" /* IWYU pragma: export */
 
 static inline char* strstr_ptr(const char *haystack, const char *needle) {