Understand augmented assignments and list modifications of __all__ variable

[tristanlatr]

Would be good to use some AST inference in order to have better output when a package is using append or extend on __all__ variable. Currently, we ignore those names, so they don't get re-exported.

[tristanlatr]

Following pyright and mypy: the following idioms should be supported for defining the values contained within __all__.
These restrictions allow pydoctor to statically determine the value of __all__.

import submodule # or from x.y import submodule
__all__ = ('a', 'b')
__all__ = ['a', 'b']
__all__ += ['a', 'b']
__all__ += submodule.__all__
__all__.extend(['a', 'b'])
__all__.extend(submodule.__all__)
__all__.append('a')

from submodule import __all__ as submod_all
__all__.extend(submod_all)