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/cryptsetup/cryptsetup-keyfile.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int find_key_file(const char *key_file, char **search_path, const char *bindname, struct iovec *ret_key);

src/cryptsetup/cryptsetup-pkcs11.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int decrypt_pkcs11_key(
                 const char *volume_name,

src/cryptsetup/cryptsetup-tokens/cryptsetup-token-util.h

@@ -4,7 +4,7 @@
 
 #include <libcryptsetup.h>
 
-#include "forward.h"
+#include "shared-forward.h"
 
 /* crypt_dump() internal indentation magic */
 #define CRYPT_DUMP_LINE_SEP "\n\t            "

src/cryptsetup/cryptsetup-tokens/cryptsetup-token.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 /* for more information see libcryptsetup.h crypt-tokens section */
 

src/cryptsetup/cryptsetup-tokens/luks2-pkcs11.h

@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int acquire_luks2_key(
                 struct crypt_device *cd,