Refactor Zig imports and file structure (part 1) (#21270)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/node_fallbacks.zig

@@ -88,10 +88,11 @@ pub fn contentsFromPath(path: string) ?string {
     return null;
 }
 
+const string = []const u8;
+
 const Fs = @import("./fs.zig");
 const std = @import("std");
 const PackageJSON = @import("./resolver/package_json.zig").PackageJSON;
-const string = @import("./string_types.zig").string;
 
 const bun = @import("bun");
 const Environment = bun.Environment;