Big package databases make GHC slow to start up

```
ezyang@sabre:~$ time ghc -no-global-package-db -e "return ()"

:1:1:
Failed to load interface for ‘Prelude’
Use -v to see a list of the files searched for.

real 0m0.100s
user 0m0.080s
sys 0m0.016s
ezyang@sabre:~$ time ghc -e "return ()"

real 0m0.230s
user 0m0.176s
sys 0m0.052s
ezyang@sabre:~$ time ghc -package-db .cabal/store/ghc-7.10.3/package.db/ -e "return ()"

real 0m0.310s
user 0m0.268s
sys 0m0.036s
```

That's a 3x difference for no discernible benefit at all! And it gets worse as the package database size increases. Maybe we should populate the plan database with exactly the packages involved in our plan.