gradle：在另一个项目下调用成绩文件

Question

5.4.1级

/project-foo/build.gradle


/project-bar/settings.gradle   
/project-bar/common-gradle/build-hello.gradle
/project-bar/common-gradle/build-world.gradle 

project-bar / common-gradle不是项目目录，而是仅包含常用的gradle文件。

project-foo / build.gradle如下调用project-bar下的build-hello.gradle：

task build_hello(type: GradleBuild) {
    dir = "${rootProject.buildDir}/build-hello"
    buildFile = file("/project-bar/common-gradle/build-hello.gradle")
    ...
}  

assemble.dependsOn build_hello

在构建project-foo时，出现错误：

Build file '/project-bar/common-gradle/hello.gradle' is not 
part of the build defined by settings file
'/project-bar/settings.gradle'. If this is an unrelated build, 
it must have its own settings file.

如果/project-bar/settings.gradle被删除，则构建project-foo即可。错误消息是什么意思？

是否应在不是项目目录的common-gradle目录下创建settings.gradle？它的内容是什么？