new-build put everything into architecture folders

Currently only `dist-newstyle/build` has an `-` subfolder. This should be extended to all `packagedb`, `cache` and `tmp` as well, I would argue.

@hvr noted, that this would break his tooling and he'd prefer to have at least
the following symlink
```
dist-newstyle/cache/plan.json -> dist-newstyle/cache/-/plan.json
```
which would point to the most recent cabal invocation.

Also of note:
- [ ] http://cabal.readthedocs.io/en/latest/nix-local-build.html?highlight=plan.json#caching would need to be updated.