How To Apply Plugin From Another Module

I have two modules app and lib.

I have added a separate file called plugin.gradle in lib module that includes a plugin as shown below

apply plugin: TestPlugin
class TestPlugin implements Plugin<Project>{

    @Override
    void apply(Project project) {
        project.task("test"){
            doLast {

               println "testing"
            }
        }
    }
}

in order to apply this plugin in lib build.gradle module ,i added the following which works perfectly fine

apply from: 'plugin.gradle'

however I can't figure out how to apply this plugin into another module (the app module)

apply from: "${project(':yourlib').projectDir}/file.gradle"

where yourlib is the module containing file.gradle