Implement fallthrough logic for options

[gdiepen]

With the merging of PR #12201, an additional layering was added to the variants in the Configuration, namely to hold the filename from which this variant was read.

This extra layer broke the original fallthrough logic of being able to overrule configuration options in the global config with back to default settings from the user config.

This PR re-implements the fallthrough logic, but now in the _get_ordered_configuration_items in the ConfigOptionParser. Instead of constructing the section_items directly as a list of key-value tuples per section, we first create a dictionary of key-value items per section (which ensures we just keep one value per key) and then convert this in one go to the original dictionary per section of a key-value list

Fixes #13696

[gdiepen]

Would be happy to add a test also, but not really sure how I would need to do that and what would work during the execution of the tests and what not.

In essence, I would like to do the exact same thing I stated in my original issue:

pip config set --global global.proxy http://non_existing_proxy_server.tld
pip config set --user global.proxy ""

pip install requests

This should result in requests being installed (or whatever package we want to test with). I did do a quick test with pip config get global.proxy but that actually shows the correct value: empty string. Just during the main program with install it goes wrong.

[notatallshaw]

Thanks for the PR, please be aware maintainer resources are fairly limited right now, doubly so as we reach the end of the year. But I will priotize reviewing this before the next release if no one else does first.