`sourcePaths` with `platform` restriction is ignored when library is used as an external dependency

[ZILtoid1991]

Works without an issue when used from subpackages, but is ignored once the package is a dependency for another one.

Offending line: https://codeberg.org/ZILtoid1991/iota/src/commit/aa4588be7abcecc2366d047fe5a3dccf22dbcc5e/dub.sdl#L22