collect the exported names in __all__

[jorenham]

if one of the exported names is an import that requires backporting it under a different name, either

• introduce an (extra) import alias for the import
• report a critical error and exit

both list and tuple literals of simple strings should be supported under assignment (=) and inplace concatenation (+=)

related:

• collect the exported names in absence of __all__ #65

relevant info:

• https://docs.python.org/3.14/tutorial/modules.html
• https://libcst.readthedocs.io/en/latest/codemods.html#libcst.codemod.visitors.GatherExportsVisitor