feat: add fs module and pkgindex custom module loading

[Sunrisepeak]

Summary

• Add C++ fs module to libxpkg runtime — provides fs.symlink, fs.readlink, fs.is_symlink, fs.entries, fs.files, fs.dirs, fs.copy_file, fs.mkdir_p, fs.remove, fs.remove_all, fs.basename, fs.dirname (all backed by std::filesystem)
• Extend import() to support xim.pkgindex.* — loads .lua modules from <pkgindex>/libs/ directory, cached after first load
• Add pkgindex_dir to ExecutionContext and set _PKGINDEX_DIR global before package loading for top-level imports

Motivation

• Package scripts (glibc, openssl, linux-headers) need filesystem primitives like symlink creation that are missing from os.* — they currently shell out to cp -r which triggers proot path resolution bugs in sandbox mode
• Package index maintainers need a way to share common code (e.g., sysroot header installation) across packages without duplicating logic in every .lua file

Test plan

• 8 new unit tests (fs module: symlink, entries, files recursive, copy_file, mkdir_p/remove; pkgindex: load from libs/, caching, unknown fallback)
• All 35 executor tests pass
• xlings builds successfully with local libxpkg