Check if new plugin's package_name is PEP 8 compliant

[Xpirix]

Fix for qgis/QGIS-Django#475

Transfered from qgis/QGIS-Django#486

Changes summary

• Update the validator for new plugin upload: raise an error if the plugin's directory (package_name) contains special characters:
  • Allowed characters: Lowercase letters (a to z), digits (0 to 9), and underscores (_).
  • First character: Must be a lowercase letter or an underscore.
  • Disallowed characters: Any special characters (e.g., -, @, #, $, etc.), uppercase letters, and whitespace.
• Update the unit tests

Uploading a new plugin with an invalid directory name will generate the following error:

Suggestion

• Ensure that the documentation (PyQGIS Cookbook) includes information about PEP8 & Python/QGIS guidelines compliance