Fix sandbox support  and some debugging conveniences

As it turns out, `setSessionDynFlags` calls `initPackage`. Unfortunately, `initPackage` only reads the package database the first time it is called. Therefore, we need to set `extraPkgConfs` before we call `setSessionDynFlags` for the first time in order for the sandbox to be used. Since `initGhci` is the first caller of `setSessionDynFlags`, we make it responsible for this.

Additionally, `build-parser` was previously assuming `bash` features despite the hash-bang only requiring `sh`.

Finally,  add a compile-time option to conveniently enable verbose output from GHC.