Fix other problems with editable installs

[abravalheri]

Motivated by pyca/cryptography use case.

Summary of changes

1. Ensure commands are finalized, to ensure plat_name is set.
2. Filter out temporary source files and prevent setuptools for attempting to consider them as part of the data_files/package_data list

Closes #3513

Pull Request Checklist

• Changes have tests -- partially. I did not manage to find a small reproducer for change 2.
• News fragment added in changelog.d/.
  (See documentation for details)