Plugins compile order in SBT - scala

Question

I'm trying to create an SBT plugin that provides common plugins and settings for my organization's projects (something like a Maven parent POM).
Since most of these settings should also be present in the commons project itself, I'm adding the sources as unmanagedSourceDirectories in plugins.sbt so the autoplugins I define for the proper project are also present on the meta-project (a neat trick I took from sbt-release plugin).
Things work just fine, except I still need to duplicate the addSbtPlugin entries when I want a plugin for booth my proper project and meta-project. If I don't do this, my proper-project won't compile since the plugins' classes are not loaded.
I tried moving all the shared plugin dependencies to a separate autoplugin assuming SBT would compile it and add the library dependencies to the meta-project so they would be there when my common plugin is compiled, but it doesn't work.
I would like to understand why this fails and if there is some way to thinker with the compilation order to somehow make it work. If not, I would gladly hear any alternatives you guys know to avoid having to maintain duplicate versions of all my shared plugins.
Here is a simplified version of my code:
project structure
/common
|-build.sbt
|-/project
| |-plugins.sbt
|-/src/main/scala/package
|-Dependencies.scala
|-MyCommonPlugin.scala
build.sbt
sbtPlugin := true
name := "common"
plugins.sbt
unmanagedSourceDirectories in Compile += baseDirectory.value.getParentFile / "src" / "main" / "scala"
Dependencies.scala
import sbt._
import sbt.Keys._
//Plugins I intend to share between build.sbt and plugins.sbt
object Dependencies extends AutoPlugin {
override def trigger = allRequirements
override lazy val projectSettings = super.projectSettings ++ Seq(
addSbtPlugin("com.typesafe.sbt" % "sbt-git" % "0.8.5")
)
}
MyCommonPlugin.scala
import sbt._
import sbt.Keys._
//this import fails! object sbt is not a member of package com.typesafe
import com.typesafe.sbt.GitBranchPrompt.{ projectSettings => gitBranchPromptSettings }
object MyCommonPlugin extends AutoPlugin {
override def trigger = allRequirements
override lazy val projectSettings =
super.projectSettings ++
gitBranchPromptSettings ++
Seq(
// My common settings
)
}
To make this code work I would need to repeat the addSbtPlugin("com.typesafe.sbt" % "sbt-git" % "0.8.5") on plugins.sbt.