When building an editable wheel it is not necessary that
`build_packages` (or even `run`) is called before calling `get_outputs`
(notice the following in
https://setuptools.pypa.io/en/latest/userguide/extension.html#supporting-sdists-and-editable-installs-in-build-sub-commands :
"Please note that custom sub-commands SHOULD NOT rely on `run()` being
executed (or not) to provide correct return values for `get_outputs()`,
`get_output_mapping()` or `get_source_files()`. The `get_*` methods
should work independently of `run().").

Our implementation relied in the call to `build_packages` to set the
name of the synthetic init file.
This commit uses a property of the object instead, to compute that name
whenever it is necessary.
With this change, it is now possible to make editable wheels.