Federated labextensions: package name logic

[martinRenou]

Description

When using the federated labextension script, the logic for finding the package name fails if the directory contains e.g. a docs directory that is a valid Python module. find_packages will find the docs module and look for the _jupyter_labextension_paths function there.
https://github.com/jupyterlab/jupyterlab/blob/master/jupyterlab/federated_labextensions.py#L373-L381

Reproduce

1. Make a docs directory that contains an __init__.py file in the extension
2. Run jupyter labextension develop . in the root directory of the extension

Expected behavior

If I can suggest a slight change in the current logic, it looks to me that the package = find_packages()[0] could be more of a "fallback" solution for when the setup.py --name is not defined, instead of the other way around.

I can come up with a PR myself if this change would be ok:

• first look for setup.py --name
• if it fails, look for find_packages()[0]

Context

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• JupyterLab version:

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[jasongrout]

I can come up with a PR myself if this change would be ok:

* first look for `setup.py --name`

* if it fails, look for `find_packages()[0]`

That sounds reasonable to me. Thanks!