Fix C++ Modules support in xmake.lua

[KeqingMoe]

The current argparse provides an xmake.lua, but for unknown reasons, it is not used by the argparse remote package in xmake-repo, and that remote package doesn't provide C++ Modules support. I tried creating a local package that could utilize argparse's existing xmake.lua, and discovered that the existing xmake.lua in argparse had some bugs regarding C++ Modules support. These bugs didn't manifest during local testing, but appeared when the package was built. This PR fixes that bug, enabling the proper use of C++ Modules when pulling the argparse package externally. After this PR is merged, I will fix the argparse package in xmake-repo to utilize the xmake.lua provided by argparse itself and provide C++ Modules support.