Make distutils symlink_data command private

[cottsay]

We don't want anyone taking a dependency on this functionality, and we don't want to interfere with non-colcon use of setuptools or distutils, so it's best to just hide this entry point and make it available only when we're running our builds.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.44%. Comparing base (b174608) to head (f46b170). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #645 +/- ## ======================================= Coverage 83.43% 83.44% ======================================= Files 66 66 Lines 3816 3817 +1 Branches 745 745 ======================================= + Hits 3184 3185 +1 Misses 555 555 Partials 77 77 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[cottsay]

Is that about correct?

Nailed it.

I discounted this as an option when it was originally implemented, but the Python metadata specification is actually pretty simple so it was easy to dump it in there. We're already modifying the PYTHONPATH when we invoke the build in order to change the installation locations via the site customization, so it isn't a very far reach to also inject some setuptools nonsense here.

All of this will be deprecated by colcon-python-project code at some point.

[nuclearsandwich]

Nailed it.

Thanks for confirming!

All of this will be deprecated by colcon-python-project code at some point.

May the day be soon at hand!