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/basic/unit-def.c

@@ -361,6 +361,17 @@ static const char* const job_mode_table[_JOB_MODE_MAX] = {
 
 DEFINE_STRING_TABLE_LOOKUP(job_mode, JobMode);
 
+/* 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);
+
 Glyph unit_active_state_to_glyph(UnitActiveState state) {
         static const Glyph map[_UNIT_ACTIVE_STATE_MAX] = {
                 [UNIT_ACTIVE]       = GLYPH_BLACK_CIRCLE,