Allow editable wheels.

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.