feat(py_info): expose VenvSymlinkEntry and VenvSymlinkKind as public APIs

[rickeylev]

Currently, VenvSymlinkEntry and VenvSymlinkKind are defined in the
private python/private/py_info.bzl file, making them inaccessible to
users who want to programmatically define virtual environment symlinks
when using PyInfo (or at the least, having to load private file
paths).

To fix this, we load and re-export both symbols in the public
python/py_info.bzl file.

Along the way

• Adds VenvSymlinkEntryBuilder to allow fluent construction of symlink
  entries via PyInfoBuilder.
• Adds features.loadable_symbols to allow programmatic detection of
  these public symbols.

[gemini-code-assist]

Code Review

This pull request exposes VenvSymlinkEntry and VenvSymlinkKind as public symbols in //python:py_info.bzl and introduces features.loadable_symbols to detect public symbols exported by bzl files. It also implements VenvSymlinkEntryBuilder to construct symlink entries and integrates it into PyInfoBuilder. The feedback suggests adding validation for the kind argument, ensuring both kind and venv_path are set before building, and clearing the _venv_symlink_builders list after building to prevent duplicate entries on multiple builder calls.

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