Improve multiline reporting

[ikonst]

Current multiline error/notice reporting is hacky, e.g.

mypy/mypy/messages.py

Lines 1161 to 1169 in c1fb57d

	self.note("Superclass:", context, offset=ALIGN_OFFSET + OFFSET, code=code)
	self.pretty_callable_or_overload(
	original,
	context,
	offset=ALIGN_OFFSET + 2 * OFFSET,
	add_class_or_static_decorator=INCLUDE_DECORATOR,
	allow_dups=ALLOW_DUPS,
	code=code,
	)

Notice how

• we call notice multiple times even as it relates to the same core issue
• we pass allow_dups=True to suppress deduplication of, say, the same type
• we mistakenly(?) dedup the "Superclass:" and "Subclass:" notices
• by always doing allow_dups=False, we can effectively dedup when it would make sense

I think it'll be better if the extra lines could be part of the error, also reducing code duplication (we end up passing a lot of the same parameters multiple times). Is there a reason why we didn't? Is it a pattern elsewhere in the code?

[hauntsaninja]

I think this would be great to do. It would help with cases like #11396 where the errors are being somewhat ad hoc reconstructed

[tusharsadhwani]

I think it'll be better if the extra lines could be part of the error

Could you clarify a bit? Current self.fail() and self.note() situation is very granular indeed, but I can't think of any way that would let us abstract it away while also preserving the current amount of flexibility.

Do we collect the error/note lines before reporting them? Do we create some class that holds the lines?

[tusharsadhwani]

@hauntsaninja Can you help clarify the solution? I can pick this up.