keys should do the right autoPlugin dance

[SethTisue]

so that e.g. this doesn't happen:

> show mimaPreviousVersion
[info] Some(1.0.6)
> set mimaPreviousVersion := None
<set>:1: error: not found: value mimaPreviousVersion
mimaPreviousVersion := None

(note set ScalaModulePlugin.mimaPreviousVersion := None works)

and that this isn't needed:

import ScalaModulePlugin._

@dwijnand describes the dance as "autoImport (or aliases, or mixed-in, or whatever)"

[dwijnand]

Reference http://www.scala-sbt.org/0.13/docs/Plugins.html#Controlling+the+import+with+autoImport

[Philippus]

If the goal is to not need the import, we would also need to move some vals (like disablePublishing) inside the autoImport object.