Plans that use bootstrapping to break cycles

In https://github.com/simonmar/happy/pull/175 (no merged) I have

```cabal
executable happy
...
if flag(bootstrap)
...
-- build-tools: happy
else
...
```
I would like to uncomment `build-tools: happy` to indicate that when bootstrapping, Happy does in fact depend on Happy, but then Cabal will complain of the cycle.

There's a lot of nuance on exactly how to make this goal, but I figured I'd start with the problem first, now that I finally have a small example of it.