refactor(import): Remove meaningless underscore imports

[MortalHappiness]

Tracking issue

N/A

Why are the changes needed?

There are many import pkg as _pkg patterns in the codebase, and many of them are meaningless. For example, there are some files containing the following weird code snippet

import os
import os as _os

flytekit/flytekit/clis/flyte_cli/main.py

Lines 3 to 4 in f1689b7

	import os
	import os as _os

which imports the os package twice.

Based on my own inference, perhaps the predecessors wanted to prevent exporting package names. However, renaming the builtin packages is meaningless.

What changes were proposed in this pull request?

This PR only deals with the import pkg as _pkg case. There are still many cases but refactoring them may introduce some unexpected behaviors. One can find these cases by searching as _ string in the codebase.

How was this patch tested?

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• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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[eapolinario]

How did you choose which ones to remove? Asking because we still have a fair amount of imports like that: https://gist.github.com/eapolinario/c340dd6fae56c25729091f616b6b1cdd.

[MortalHappiness]

@eapolinario I choose the imports with import pkg as _pkg pattern. I do not remove those imports like import nested.pkg as _pkg or from pkg import obj as _obj.

I do not remove those imports because I do not know the context why the predecessors rename the imports by adding the underscores as I mentioned in the PR description. Perhaps they want to keep them private? I am afraid that my changes will break some functionalities, so only those obviously meaningless imports are deleted.