Merge shared/exec-directory-util.? into basic/unit-def.?

Suggested in
https://github.com/systemd/systemd/pull/35892#discussion_r2180322856.

This is a tiny amount of code and does not warrant having a separate file
and spawning a separate instance of the compiler during the build.

Note: it took me a while to confirm that the contents of that table and
function don't end up in libsystemd.so. The issue is that they _are_ present in
it, unless LTO is used. We actually use link_whole[libbasic_static] for
libsystemd, so we end up with all that code there. LTO is needed to clean
that up.

src/shared/exec-directory-util.c

@@ -1,15 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-
-#include "exec-directory-util.h"
-#include "string-table.h"
-
-/* This table maps ExecDirectoryType to the setting it is configured with in the unit */
-static const char* const exec_directory_type_table[_EXEC_DIRECTORY_TYPE_MAX] = {
-        [EXEC_DIRECTORY_RUNTIME]       = "RuntimeDirectory",
-        [EXEC_DIRECTORY_STATE]         = "StateDirectory",
-        [EXEC_DIRECTORY_CACHE]         = "CacheDirectory",
-        [EXEC_DIRECTORY_LOGS]          = "LogsDirectory",
-        [EXEC_DIRECTORY_CONFIGURATION] = "ConfigurationDirectory",
-};
-
-DEFINE_STRING_TABLE_LOOKUP(exec_directory_type, ExecDirectoryType);

src/shared/exec-directory-util.h

@@ -1,19 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-#pragma once
-
-#include <errno.h>
-
-#include "macro-fundamental.h"
-
-typedef enum ExecDirectoryType {
-        EXEC_DIRECTORY_RUNTIME,
-        EXEC_DIRECTORY_STATE,
-        EXEC_DIRECTORY_CACHE,
-        EXEC_DIRECTORY_LOGS,
-        EXEC_DIRECTORY_CONFIGURATION,
-        _EXEC_DIRECTORY_TYPE_MAX,
-        _EXEC_DIRECTORY_TYPE_INVALID = -EINVAL,
-} ExecDirectoryType;
-
-const char* exec_directory_type_to_string(ExecDirectoryType i) _const_;
-ExecDirectoryType exec_directory_type_from_string(const char *s) _pure_;

src/shared/meson.build

@@ -69,7 +69,6 @@ shared_sources = files(
         'elf-util.c',
         'enable-mempool.c',
         'ethtool-util.c',
-        'exec-directory-util.c',
         'exec-util.c',
         'exit-status.c',
         'extension-util.c',