gnulib-tool.py: Fix pylint 'attribute-defined-outside-init' warnings.

[Collin Funk]

I mentioned a change involving moving an attribute definition to its
proper place in __init__ here:

    https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00270.html

But I noticed that pylint will warn about these [1]. So it seems
easier to fix them all at once instead of in the middle of other
changes.

$ pylint *.py | grep W0201
GLError.py:119:12: W0201: Attribute 'message' defined outside __init__ 
(attribute-defined-outside-init)
GLImport.py:1043:8: W0201: Attribute 'assistant' defined outside __init__ 
(attribute-defined-outside-init)
GLModuleSystem.py:916:8: W0201: Attribute 'modules' defined outside __init__ 
(attribute-defined-outside-init)

GLError seems like it was never completed, since we have the error
messages repeated under "if __name__ == '__main__':". But it seems
__repr__ expects 'messages' to be initialized to None. So we can just
do that now in __init__.

GLImport defines 'assistant' in GLImport.execute(). I plan to remove
that definition and just mutate an instance attribute. Before that
change, it is harmless to initialize it to a GLFileAssistant in
__init__ so lets do that. We already unnecessarily create a
GLFileSystem that we don't need to [2] [3]. :)

The GLModuleSystem.transitive_closure() function defines a 'modules'
attribute that is never used. We can just remove it.

[1] 
https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/attribute-defined-outside-init.html
[2] 
https://git.savannah.gnu.org/cgit/gnulib.git/tree/pygnulib/GLImport.py?id=64a7072ee8636c53bc2000505668bd0e33bb427e#n305
[3] 
https://git.savannah.gnu.org/cgit/gnulib.git/tree/pygnulib/GLImport.py?id=64a7072ee8636c53bc2000505668bd0e33bb427e#n1134

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0001-gnulib-tool.py-Fix-pylint-attribute-defined-outside-.patch
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