Raise an error if .root() is not a module

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore)

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.root() should guarantee that what it returns is a module. Unfortunately, I found a couple of cases, which involved transforms, where .root() return classes instead of modules. We should raise an error in that case (and probably we should fix whatever was returning a class to return a module instead).

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/120

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

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Actually this can happen if the tree was spliced from another and it is in the course of being transformed.

[temyurchenko]

Why is this completed? ".root" sometimes returns not a module. In particular, for some nodes made with const_factory

[Pierre-Sassoulas]

#2672 gave an example of root not being a module but Unknown instead.

a=eval.__get__(1).__gt__

@a
class c: ...