Re-organize exports for new validators and serializers

[adriangb]

Selected Reviewer: @samuelcolvin

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[samuelcolvin]

I'm totally in agreement about moving methods to the new modules that make sense, but I really don't think we can remove the methods from __init__.py.

1. I think knowing that everything in the public API can be imported from pydantic is really helpful
2. One line imports for pydantic use is much cleaner and simpler
3. changing this now, especially when we're adding new recommended functions like field_validator will add a lot more churn to the migration

On balance I think this is one of those "correct" vs. "easy" decisions where I tend to lean towards easy and you tend to lean towards correct.

I think on this occasion we should revert that change.

The only exception to the "everything in __init__.py" rule right now (definitely the only important one) is dataclass - on this I think we should:

1. re-export our version of dataclass
2. rename the dataclasses module to pydantic_dataclasses to avoid confusion with std-lib dataclasses module.

[adriangb]

If we're going to export something via pydantic/__init__.py I don't think there's much point in also exporting it from another module. Right?

[samuelcolvin]

I think there is - some people might well want to work like

from pydantic import validators

...
@validators.[tab completion]

More to the point, to "no export from another module" would require all other pydantic modules to be made private, which seems a little ugly.

[adriangb]

please review (will solve conflicts)

[samuelcolvin]

otherwise LGTM.

[samuelcolvin]

root_validator should surely move into validators.py?

More generally can we remove this module, or move it to _internal.

[adriangb]

I didn't move it because:

1. Backwards compatibility with V1.
2. We want to deprecate @root_validator anyway

[samuelcolvin]

root_validator and validator where in class_validators.py in V1, the logic for supporting imports from the old path is already supported.

I think we move them.

[samuelcolvin]

I have a fix for the above issues. Please don't push until I've pushed my fix - currently can't due to problems with GH.