Parse paths as set rather than list

[pswaminathan]

Occasionally a directory will be included multiple times. This can happen when the includes is underneath the root path for the sources. e.g. https://github.com/TimOliver/TOCropViewController/tree/main/Objective-C/TOCropViewController. In many cases this may be acceptable. But if a module.modulemap file is included twice, it shouldn't fail.