Unifies AutoPlugin and AutoImport. Fixes #1188

* AutoImport trait is subsumed by def autoImport method under
AutoPlugin class.
* When def autoImport is overridden by a lazy val or a val, *.sbt
automatically imports autoImport._.
Adjusting to 0.13 changes
Remove def autoImport: Any from AutoPlugin.
Merge pull request #1204 from eed3si9n/topic/valAutoImport

Unifies AutoPlugin and AutoImport. Fixes #1188