ProgramDb should be transparently lazy, or not lazy at all

Why does the type signature of `requireProgram` return a `ProgramDb`?

```
requireProgram :: Verbosity -> Program -> ProgramDb
-> IO (ConfiguredProgram, ProgramDb)
```

The answer is, if a program was previously not configured, and `requireProgram` configures it, then it is now recorded as a configured program in the new ProgramDb.

Sounds good, right? No! Grep Cabal's source code and you'll see plenty of places where people `requireProgram` and ignore the returned `ProgramDb`. This just seems like a work around the fact that `ProgramDb` is not a mutable data structure. (It's convenient for this to be pure since we serialize it.) We should try to do something better here! I think some sort of mutable variant of `ProgramDb` which automatically "fixes itself up" would be better,