Add global hook point for get_build_system_dependencies

[vshawrh]

Problem

setuptools 81 removed distutils.spawn(dry_run=...) and distutils.dir_util.remove_tree(dry_run=...). setuptools 82 removed pkg_resources entirely. Many packages on PyPI still use these in their setup.py, causing build failures when Fromager resolves an uncapped setuptools as a build dependency.

In the AIPCC wheels builder (fondue), 22 identical per-package plugins exist solely to add a setuptools version cap via get_build_system_dependencies. Each plugin is a copy-paste of the same logic. Additionally, ~127 packages with settings overrides have no protection at all.

Proposed Solution

Instead of putting setuptools-specific detection logic into Fromager core, extend fromager.hooks to support get_build_system_dependencies as a global hook point. This lets downstream projects register hooks that post-process build dependencies for all packages.

Changes in Fromager (~20 lines across two files)

hooks.py:

• Add get_build_system_dependencies to GLOBAL_HOOK_NAMES
• Add run_get_build_system_dependencies_hooks() following the same pattern as run_post_build_hooks()
• The hook receives the resolved requirements list and returns a modified list

dependencies.py:

• Inside get_build_system_dependencies(), after overrides.find_and_invoke() returns the dependency list (from a per-package plugin or the default), pass it through the global hook before _filter_requirements()

How downstream projects use it

Downstream projects register hooks via fromager.hooks entry points, same as post_build today:

[project.entry-points."fromager.hooks"]
get_build_system_dependencies = "my_plugins.hooks.setuptools_cap:get_build_system_dependencies"

The hook signature:

def get_build_system_dependencies(
    ctx: context.WorkContext,
    req: Requirement,
    sdist_root_dir: pathlib.Path,
    build_dir: pathlib.Path,
    requirements: list[str],
) -> list[str]:
    # post-process and return modified requirements
    ...

Why this approach

• Fromager stays generic -- no setuptools version numbers or AST scanning in core
• Opt-in -- users register hooks or don't. No forced behavior
• Extensible -- downstream projects can add any build dependency fixups (setuptools caps, cmake constraints, custom build deps) without Fromager core changes
• Follows existing patterns -- same mechanism as post_build / post_bootstrap / prebuilt_wheel hooks

Impact

In the fondue builder:

• 22 plugins eliminated -- one global hook replaces all of them
• 3 plugins simplified (llvmlite, torchvision, pyarrow) -- can remove setuptools constraint handling from their get_build_system_dependencies override
• ~127 packages gain protection -- currently have no setuptools cap

Previous approach

PR #1264 (now closed) put the setuptools detection logic directly in default_get_build_system_dependencies. Per feedback from @tiran, this was too specific to put in core. The global hooks approach keeps Fromager generic while giving downstream projects the extension point they need.

Relationship to existing issues

Related to #928 (Reduce downstream plugin needs through YAML-driven configuration enhancements).