Unable to import a scala object in build.sbt

Question

I have a scala file called ConcatPlugin.scala in package com.mariussoutier.sbt.

However when I import it in build.sbt I get error cannot resolve symbol ConcatPlugin

Build.sbt

import com.mariussoutier.sbt.ConcatPlugin

But when I type it in it does show that the class exists.

The ConcatPlugin.scala file can be found here