Before packages did not had to set `format = "other";` if had a custom
installPhase and would not produce a dist folder.
The current error message is hard to understand by new users:
> Executing pythonOutputDistPhase
> mv: cannot stat 'dist': No such file or directory
This commit improves the error message.
Store the intermediate artifacts. In time, we should build, install and
test in separate derivations as that reduces circular dependencies,
avoids rebuilds when fixing tests, and makes it possible to use the
wheels for creating say virtualenv's.