Declare default imports in pydanticgen/pydanticgen.py

[Silvanoc]

PR #2329 is reducing some parts of the Pydantic generator to generate the imports. Once that PR gets merged, it would be meaningful to centralize the declaration of default imports and additional import sets.

          can we move the declaration of the default imports out of this method? eg. in pydanticgen it's at the module level: https://github.com/linkml/linkml/blob/c4379e5a121732b6adda21284bb34dde590743b1/linkml/generators/pydanticgen/pydanticgen.py#L70

Originally posted by @sneakers-the-rat in #2329 (comment)

[sneakers-the-rat]

maybe not centralize, since the pythongen imports are pretty different than the pydanticgen ones, but move from the method definition to the module yes yes :)