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-table.h

@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include "forward.h"
+#include "basic-forward.h"
 
 const char* string_table_lookup_to_string(const char * const *table, size_t len, ssize_t i) _const_;