Unify field names in queryset classes

[henadzit]

Description

This is a refactoring change.

This PR:

• Replaces duplicate fields with a single field in AwaitableQuery and its subclasses, e.g. instead of having both _annotations in AwaitableQuery (the parent) and annotations in UpdateQuery (the child), there will be just a single field called _annotations
  • This is mostly a renaming change but I had to update the logic of _make_query slightly because there was logic that relied on having two distinct fields _annotations and annotations.
• Make most of query fields private to simplify queryset interface
• Add tests for the cases where the query is modified and reused multiple times

Motivation and Context

When I was debugging the queryset code, it was really hard to understand what is going on because of the fields named almost identically but having different undocumented purposes. Reusing the fields should make the code more readable and easier to debug.

Here's a relevant discussion in another pull request #1748 (comment)

How Has This Been Tested?

• Tested the package locally with a sample web app
• Tests

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 11561852158

Details

• 141 of 146 (96.58%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.006%) to 89.018%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/queryset.py	141	146	96.58%

Files with Coverage Reduction	New Missed Lines	%
tortoise/queryset.py	1	94.54%

Totals
Change from base Build 11529966516:	-0.006%
Covered Lines:	5981
Relevant Lines:	6604

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💛 - Coveralls

[henadzit]

@abondar, would love to hear your feedback on this one!

[abondar]

Yeah, thank you for this refactoring, it really was confusing, but never had time to take it on