Rename root project to sbtRoot.
Rename root project to sbtRoot.

This is mostly for IntelliJ IDEA.

Currently IntelliJ IDEA's Scala (and SBT) plugin defines:

* the project name (as seen in the window title and in the "open recent
  project" list) from `name`
* the root module (as seen in the project view and in project structure)
  from `id`
* doesn't use `moduleName` at all

After this change the sbt project is no longer identified as "root".

I was undecided between `sbtRoot` and `sbtRootProj`, and went with the
shorter option. I'm happy to revise this decision.

mdedetrich/sbt,xuwei-k/xsbt,sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,mdedetrich/sbt,Duhemm/util,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt