If a package build-depends on an executable only package, we repeatedly rebuild it

[ezyang]

Steps to reproduce:

1. Put happy in build-depends of a package
2. cabal new-build it (download and install happy, then fail)
3. cabal new-build it again

Expected result: fails immediately, without rebuilding happy

Actual result: rebuild happy first

Ostensibly we need to look to see if there's an appropriately named directory in the store when we improve. But there's also a refactoring where PreExisting ipkg is insufficient to represent executables (which don't have installed package infos).

[ezyang]

This is fixed in HEAD.