align typing with pandas source

[Dr-Irv]

Possibly closes #128

I went through the current pandas source in pandas/_typing.py and separated the types declared there from the "private" types only used in the stubs, which now appear at the bottom of the file, and added documentation where appropriate to indicate where things are different.

Unclear whether the "private" types in the stubs should all be preceded by underscores. They are imported from pandas/_typing, so that makes them private anyway. When I wrote #128, I suggested to make them all private. Now that I have more experience with typing, I don't think it's necessary.

By doing this, I have a list of types that should be "public" which will eventually go in pandas.api.typing.aliases . That will then allow me to work on pandas-dev/pandas#55231

Asking for opinions of @loicdiridollou and a review/approval/merge from @twoertwein

[loicdiridollou]

Great job in here, I think this is a major improvement. Hopefully that can unblock the other issue in pandas repo.
Added a few comments about some types being defined in files instead of _typing.pyi.
Not totally sure what is the exact direction that would fit the team at pandas but this is a ton of work already here and we can continue to use the stubtest to clean up some of the function signatures that have drifted from the pandas implementaion.

[Dr-Irv]

Added a few comments about some types being defined in files instead of _typing.pyi.

For now, I'd rather mirror what is in the pandas source, where both of those types are defined in the respective files. That's where I picked them up from. If someone really wants access to them, they can import them from those files, which are not in private modules.

[Dr-Irv]

@twoertwein thanks for the review. I've pushed a new version that addresses your comments (and commented on a few other comments where I didn't make a change)

[twoertwein]

Thanks @Dr-Irv !