It's kind of an inverted dependency, because org.eclipse.swt.win32.win32.x86
is a fragment bundle that is hosted by the base org.eclipse.swt
bundle and extends its API. The SWT base bundle is unusual because it contains no code at all... everything is in the platform-specific fragments.
This works in PDE because the base bundle contains the header "Eclipse-ExtensibleAPI: true". When PDE sees this header, it searches for fragments that can add API to the base bundle. Sadly if you are writing some kind of build process to automate PDE builds you will need to replicate the same logic.
Fortunately, at build time you don't need to worry about which platform you are compiling for: the signatures of all the classes and methods are the same across the supported platforms, so you can easily compile using the Win32 fragment even if you are compiling on a Mac (for example).