refactor(python): Rename utils module to _utils to explicitly mark it as private

[stinodego]

As discussed on Discord.

In our deprecation policy, we explicitly state that changing internal module paths is not considered a breaking change. To avoid surprises for users that do not read the policy, I am explicitly marking the utils module as private.

I did notice that the _get_shared_lib_location util is used in the plugins docs. It is not used internally. What is the deal with this (private) function and should we make it public? @MarcoGorelli would appreciate your input!

[MarcoGorelli]

I had it in my backlog to ask about making _get_shared_lib_location public, so thanks for bringing this up 😄

For plugins, the other thing I'd love would be a public parse_into_expr function - currently I've put this one in the cookiecutter, which only uses public methods, but it would be nice to just do

from polars.plugin_utils import (
    get_shared_lib_location,
    parse_into_expr,
)

https://github.com/MarcoGorelli/cookiecutter-polars-plugins/blob/6071d14e88deebea4c73cbbb8d63b7d09d9f1da1/%7B%7B%20cookiecutter.project_slug%20%7D%7D/%7B%7B%20cookiecutter.project_slug%20%7D%7D/utils.py#L11-L48

---

Having said that, with this change, all plugins will break, even though authors just followed official guidance. I'd really appreciate it if

from polars.utils.udfs import _get_shared_lib_location

could keep working for now, but just emit a deprecation warning, advising to use whatever the replacement path is. Given that polars.api already has register_expr_namespace, maybe that would be a good place for this to live as well?

[codecov]

Codecov Report

Attention: Patch coverage is 93.44262% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 80.93%. Comparing base (d482bce) to head (e2944ab).
Report is 2 commits behind head on main.

❗ Current head e2944ab differs from pull request most recent head dccec10. Consider uploading reports for the commit dccec10 to get more accurate results

Files	Patch %	Lines
py-polars/polars/_utils/udfs.py	91.62%	19 Missing and 12 partials ⚠️
py-polars/polars/utils/udfs.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14772      +/-   ##
==========================================
- Coverage   80.94%   80.93%   -0.01%     
==========================================
  Files        1324     1325       +1     
  Lines      171941   171904      -37     
  Branches     2450     2452       +2     
==========================================
- Hits       139176   139129      -47     
- Misses      32296    32305       +9     
- Partials      469      470       +1     

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[stinodego]

Having said that, with this change, all plugins will break, even though authors just followed official guidance.

All right, will keep that unchanged in this PR. Can follow up with a PR to add a polars.plugins module with some of the relevant utils.

[MarcoGorelli]

awesome, thanks! I'll repurpose #14779 as a follow-up then once this is in