by hackattack142 » 13. July 2012 23:19
The short answer is it is included in the download and there is not a code-specific repository (that I am aware of at least)
The Control Panel and Installer source is in the XAMPP download (see the src folder).
The various batch scripts are plaintext and can just be read in a text editor.
The modules (Apache, PHP, etc) were not modified and their source can be found at their respective sites.
The XAMPP-specific module configuration changes can be found by comparing a vanilla configuration files with the XAMPP ones.