title: SBT: Invalid project ID: Expected ID character when pointing to module in subdirectory
body: <p>I a multi-module project structured like this</p>
  
  <pre><code>+- multiModuleProject
    +-module1
  
    +-dir1
      +-subDirModule1
      +-subDirModule2
  
    +-module3
  
    +-build.sbt
  </code></pre>
  
  <p>I want both <code>subDirModule1</code> and <code>subDirModule2</code> to be their own modules outright.</p>
  
  <p>I added something like this to the <code>build.sbt</code></p>
  
  <pre><code>lazy val subDir1 = Project(id = "dir1/subDirModule1", base = file("dir1/subDirModule1")
  lazy val subDir1 = Project(id = "dir1/subDirModule2", base = file("dir1/subDirModule2")
  </code></pre>
  
  <p>I can't get it to work, I keep getting </p>
  
  <pre><code>[error] java.lang.RuntimeException: Invalid project ID: Expected ID character
  [error] dir1/subDirModule1
  [error]     ^
  </code></pre>
  
  <p>But I'm sure I've seen a slash being used in another project I've worked on. What going wrong here?</p>