Incorrect `no-member` linter errors from cython modules

Follow-up from "Refactor: Lazily instantiate ProvenanceInformation objects"

The following discussion from !2026 (merged) should be addressed:

• @BenjaminSchubert started a discussion: (+1 comment)

  @tristanvb I've look into the linting error more, I can't figure out why it does fail.

  At that point I'd suggest extracing the top_element.link_target.as_str() in a variable and silence pylint for that error on the line

Follow up comment

I chased this down the rabbit hole a bit, and stumbled on this post, and discovered we are whitelisting our cython modules to be loaded by the linter and telling the linter to enable c-extension-no-member.

I was also unable to get to the bottom of it, it seems we are hitting some kind of edge case, and I think a hint is that the errors themselves are incorrect, if I remove all the disabling of no-member in loader.py:

src/buildstream/_loader/loader.py:149:8: E1101: Class 'dependencies' has no 'extend' member (no-member)
src/buildstream/_loader/loader.py:354:26: E1101: Class 'link_target' has no 'as_str' member (no-member)
src/buildstream/_loader/loader.py:392:27: E1101: Class 'node' has no 'get_str' member (no-member)

In each case, the linter is mistaking a variable name for a class, the error in question should be printed as:

src/buildstream/_loader/loader.py:354:26: E1101: Class 'ScalarNode' has no 'as_str' member (no-member)

But the linter instead thinks the reference to the instance link_target is itself the ScalarNode class.

In any case, for now I will just disable the error as you suggest.