chore: support removal of builtin providers

[comius]

The builtin providers PyInfo, PyRuntimeInfo, and PyCcLinkParamsProvider are being removed,
which means Bazel throws an error while compiling bzl files if there is a reference to a
top-level symbol that doesn't exist anymore. For backwards compatibility, rules_python
consumes/produces these providers, so the symbols are used in various places.

To fix, use native.legacy_globals and Bazel version detection to conditionally emit
the symbols into @rules_python_internal. If they aren't present, they are reported
as None.

This mimics equivalent functionality in bazel_features; bazel_features isn't used because
it would require users to update their WORKSPACE to initialize some dependencies before
rules_python can perform its initialization.

Removal of the builtin symbols is controlled by --incompatible_autoload_externally
(which is in Bazel 8 and has been cherry-picked into earlier version). If the flag is
enabled with "@rules_python" or "-@rules_python" the providers are removed from Bazel.

[aignas]

I am somewhat opposed to having a breaking change to our WORKSPACE users, but looking at the code I don't see a quick workaround for that.

I am wondering if there is a different way to achieve this? I looked at the repos that we actually need from bazel_features and it seems that under WORKSPACE there is no way to initialize them inside the py_repositories call. So there is definitely no way but to have this extra call done after py_repositories(). I think having bazel-features in the long term could be inevitable and could make the code cleaner, but the fact that all of the WORKSPACE users would need to do these additions is unfortunate.

That said, everyone should migrate to bzlmod anyway, so this could be a good push to do this. Any other opinions? We at least need a decent note in the CHANGELOG.md explaining the migration steps.

[rickeylev]

Is there a reason it can't go in internal_config_repo? I didn't fully trace what bazel_features is doing, but it looked like it was just generating code like:

globals = struct(
  PyInfo = getattr(getattr(native, "legacy_globals", None), "PyInfo", None)

That looks fairly simple to drop into python/private/internal_config_repo.bzl

[rickeylev]

I updated the rules_python_internal repo to do the globals detection and updated reexports.bzl.

I think its working? I think bazel@head still isn't quite ready, so wasn't able to fully test with autoloading enabled (got some undefined symbol errors for stuff in bazel_tools, but those were also present with the PR as it was before I made my modifications). I didn't have a chance to lookup how the autoloading flag works, either, so maybe I set it wrong.

I'm not sure I'll have time tonight to undo the py_repositories_deps() changes.

[rickeylev]

Ok, finished cleanup etc. I think this is good to go