Move libzodium files to a dedicated directory

build.zig

@@ -16,7 +16,7 @@ pub fn build(b: *std.Build) !void {
     minisign.linkLibC();
     if (use_libzodium) {
         const libzodium_mod = b.createModule(.{
-            .root_source_file = b.path("src/libzodium.zig"),
+            .root_source_file = b.path("src/libzodium/libzodium.zig"),
             .target = target,
             .optimize = optimize,
         });

src/helpers.c

@@ -15,7 +15,7 @@
 #include <string.h>
 
 #ifdef LIBZODIUM
-#    include "zodium.h"
+#    include "libzodium/sodium.h"
 #else
 #    include <sodium.h>
 #endif

src/minisign.c

@@ -11,7 +11,7 @@
 #include <unistd.h>
 
 #ifdef LIBZODIUM
-#    include "zodium.h"
+#    include "libzodium/sodium.h"
 #else
 #    include <sodium.h>
 #endif