Rename basic-forward.h, sd-forward.h, and shared-forward.h to forward.h

Follow-up for 74d392ed1b.

docs/CODING_STYLE.md

@@ -302,9 +302,9 @@ SPDX-License-Identifier: LGPL-2.1-or-later
     inline functions that require the full definition of a struct into the
     implementation file so that only a forward declaration of the struct is
     required and not the full definition.
-  - `src/basic/basic-forward.h` contains forward declarations for common types.
-    If possible, only include `basic-forward.h` in header files which makes
-    circular header dependencies a non-issue.
+  - `src/basic/forward.h` contains forward declarations for common types. If
+    possible, only include `forward.h` in header files which makes circular
+    header dependencies a non-issue.
 
   Bad:
 
@@ -382,10 +382,9 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 
   For common code, there are three different forward declaration headers:
 
-  - `src/basic`: `basic-forward.h`
-  - `src/libsystemd`: `sd-forward.h`
-  - `src/libsystemd-network`: `sd-forward.h`
-  - `src/shared`: `shared-forward.h`
+  - `src/basic/forward.h`
+  - `src/libsystemd/forward.h`
+  - `src/shared/forward.h`
 
   Header files that extend other header files can include the original header
   file. For example, `iovec-util.h` includes `sys/uio.h`. To identify headers
@@ -412,7 +411,7 @@ SPDX-License-Identifier: LGPL-2.1-or-later
   ```c
   // source.h
 
-  #include "basic-forward.h"
+  #include "forward.h"
 
   void my_function_that_logs(size_t sz);