InstallPlan: Add a Processing package state.
InstallPlan: Add a Processing package state.

We don't want 'InstallPlan.ready' to return packages that are currently
being processed in other threads.

corngood/cabal,anton-dessiatov/cabal,themoritz/cabal,eric-stanley/cabal,christiaanb/cabal,bennofs/cabal,plumlife/cabal,rimmington/cabal,trskop/cabal,themoritz/cabal,thomie/cabal,plumlife/cabal,mightybyte/cabal,sopvop/cabal,fpco/cabal,edsko/cabal,mydaum/cabal,Helkafen/cabal,Helkafen/cabal,enolan/cabal,themoritz/cabal,fugyk/cabal,valderman/cabal,triplepointfive/cabal,kosmikus/cabal,Heather/cabal,typelead/epm,thoughtpolice/cabal,garetxe/cabal,kolmodin/cabal,headprogrammingczar/cabal,fugyk/cabal,beni55/cabal,Heather/cabal,valderman/cabal,trskop/cabal,seereason/cabal,lukexi/cabal,mightybyte/cabal,headprogrammingczar/cabal,gridaphobe/cabal,Peaker/cabal,triplepointfive/cabal,ian-ross/cabal,gbaz/cabal,seereason/cabal,gbaz/cabal,randen/cabal,ghc-android/cabal,ghc-android/cabal,martinvlk/cabal,bennofs/cabal,plow-technologies/cabal,lukexi/cabal,ian-ross/cabal,kolmodin/cabal,enolan/cabal,thomie/cabal,thoughtpolice/cabal,sopvop/cabal,christiaanb/cabal,x-y-z/cabal,rimmington/cabal,typelead/epm,mmakowski/cabal,anton-dessiatov/cabal,beni55/cabal,x-y-z/cabal,mydaum/cabal,gridaphobe/cabal,edsko/cabal,garetxe/cabal,randen/cabal,fpco/cabal,Peaker/cabal,martinvlk/cabal,mmakowski/cabal,sopvop/cabal,corngood/cabal,eric-stanley/cabal,plow-technologies/cabal,kosmikus/cabal