The module name comes from the Product Module Name build setting:
The SWIFT_MODULE_NAME
setting is apparently hidden, but you can see its derivation by looking at Xcode.app/Contents/PlugIns/Xcode3Core.ideplugin/Contents/SharedSupport/Developer/Library/Xcode/Plug-ins/XCLanguageSupport.xcplugin/Contents/Resources/Swift.xcspec
:
...
{
Name = "SWIFT_MODULE_NAME";
Type = String;
DefaultValue = "$(PRODUCT_MODULE_NAME)";
CommandLineArgs = (
"-module-name",
"$(value)",
);
},
...