xcworkspace

workspace, err := Open("workspace.xcworkspace")
if err != nil {
	panic(err)
}

var fileRefLocations []string
for _, fileRef := range workspace.FileRefs {
	pth, err := fileRef.AbsPath("workspace_dir")
	if err != nil {
		panic(err)
	}

	fileRefLocations = append(fileRefLocations, pth)
}
for _, group := range workspace.Groups {
	groupPth, err := group.AbsPath("workspace_dir")
	if err != nil {
		panic(err)
	}

	for _, fileRef := range group.FileRefs {
		pth, err := fileRef.AbsPath(groupPth)
		if err != nil {
			panic(err)
		}

		fileRefLocations = append(fileRefLocations, pth)
	}
}

var projects []xcodeproj.XcodeProj
for _, fileRefLocation := range fileRefLocations {
	if !xcodeproj.IsXcodeProj(fileRefLocation) {
		continue
	}

	project, err := xcodeproj.Open(fileRefLocation)
	if err != nil {
		panic(err)
	}
	projects = append(projects, project)
}

Output: