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.