Introduce a companion `Incremental` class.
Introduce a companion `Incremental` class.

Move most of the functionality from `Incremental` object to its
companion class.

This commit is a preparation for making it possible to have
two different implementation of logic in `Incremental` object.

sbt/sbt,sbt/sbt,mdedetrich/sbt,jasonchaffee/sbt,twitter-forks/sbt,xuwei-k/xsbt,Duhemm/compiler-bridge,corespring/sbt,jaceklaskowski/sbt,jkinkead/sbt,jaceklaskowski/sbt,adriaanm/sbt,xeno-by/old-scalameta-sbt,mdedetrich/sbt,twitter-forks/sbt,mdedetrich/sbt,xuwei-k/xsbt,jkinkead/sbt,xeno-by/old-scalameta-sbt,pdalpra/sbt,jkinkead/sbt,jkinkead/sbt,xuwei-k/xsbt,dyx/sbt,twitter-forks/sbt,jaceklaskowski/sbt,xeno-by/old-scalameta-sbt,dyx/sbt,mdedetrich/sbt,xuwei-k/xsbt,xeno-by/old-scalameta-sbt,dyx/sbt,dyx/sbt,sbt/sbt,dansanduleac/sbt,jaceklaskowski/sbt,sbt/sbt,mdedetrich/sbt,dyx/sbt,jkinkead/sbt,sbt/sbt,xeno-by/old-scalameta-sbt,mdedetrich/sbt,twitter-forks/sbt,sbt/sbt,twitter-forks/sbt,jaceklaskowski/sbt,som-snytt/xsbt,Duhemm/sbt,xeno-by/old-scalameta-sbt,Duhemm/sbt,sbt/sbt,mdedetrich/sbt,Sciss/sbt,pocketberserker/sbt,xuwei-k/xsbt