New in version 3.15.
Specify output directory for Swift modules provided by the target.
If the target contains Swift source files, this specifies the directory in which the modules will be placed. When this property is not set, the modules will be placed in the build directory corresponding to the target’s source directory. If the variable CMAKE_Swift_MODULE_DIRECTORY is set when a target is created its value is used to initialise this property.
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https://cmake.org/cmake/help/v3.19/prop_tgt/Swift_MODULE_DIRECTORY.html