refactor(py)!: encapsulate dotpromptz internal modules behind private prefixes

[huangjeff5]

New code imports from dotpromptz. Published genkit still imports the old module paths. Those imports do not warn.

# new code
from dotpromptz import DataArgument, DirStore, Dotprompt, Message

# still works for pip install genkit (0.12.0), no warning
from dotpromptz.dotprompt import Dotprompt
from dotpromptz.typing import DataArgument, PromptFunction, PromptInputConfig, PromptMetadata

Decisions

• Published genkit 0.12.0 still imports the old modules, silently: pip install genkit depends on dotpromptz>=0.1.5 with no ceiling and imports dotpromptz.dotprompt and dotpromptz.typing. Those modules, plus stores, errors, helpers, parse, picoschema, resolvers, and util, re-export the private modules and print nothing. We chose silence over a DeprecationWarning because that import lives inside published genkit, and the app cannot change it. Remove the modules once genkit no longer installs this package that way.
• Dotprompt's public signatures import from the root: compile() returns PromptFunction, and the constructor takes HelperFn, EscapeFunction, JsonSchema, PromptInputConfig, ModelConfigT, and VariablesT. Those names are exported from dotpromptz.
• Root dotpromptz is the door for new code: Implementation modules are _dotprompt, _typing, _stores, and the other _ modules. New callers import from dotpromptz, not from those files.

[pavelgj]

A few comments/questions

[pavelgj]

LGTM with comments

[huangjeff5]

@gemini review