feat: add get_build_system_dependencies global hook point

[vshawrh]

Summary

Adds get_build_system_dependencies as a new global hook point in fromager.hooks. This lets downstream projects register hooks that post-process build-system dependencies for all packages, without needing per-package plugins.

Motivation: In the AIPCC/fondue builder we have 22+ identical per-package plugins that exist solely to cap setuptools for packages whose setup.py uses removed APIs (pkg_resources, dry_run). With this hook point, we can replace all of them with a single global hook.

Changes

• src/fromager/hooks.py: Added get_build_system_dependencies to GLOBAL_HOOK_NAMES and implemented run_get_build_system_dependencies_hooks() which chains hooks so each receives the (possibly modified) requirements list from the previous one.
• src/fromager/dependencies.py: After overrides.find_and_invoke() returns build-system deps and before _filter_requirements(), the global hooks are now called to allow post-processing.
• tests/test_hooks.py: Tests for the new hook: basic invocation, chaining behavior, no-op when no hooks registered, exception propagation.
• tests/test_dependencies.py: Integration test verifying get_build_system_dependencies() calls the global hooks.

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]:
    ...

Registered via entry points under fromager.hooks:

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

How it fits

The hook runs after per-package overrides (overrides.find_and_invoke) and before marker filtering (_filter_requirements). This means per-package plugins still take full precedence, and global hooks only augment the result.

Closes #1263

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a global get_build_system_dependencies hook and runner that chains registered plugin results. Build-system dependency resolution now invokes this hook after initial resolution and before filtering and writing requirements. Tests cover argument forwarding, hook chaining, empty hook registration, exception propagation, and end-to-end integration with dependency resolution.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change adds the requested global hook, chains it, and invokes it after per-package overrides before filtering.
Out of Scope Changes check	✅ Passed	The PR stays focused on the new hook, integration, and tests; no unrelated changes are evident.
Title check	✅ Passed	The title clearly summarizes the main change: adding a new global hook point for build-system dependencies.
Description check	✅ Passed	The description is directly related to the changeset and accurately describes the new hook, integration, and tests.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@tests/test_dependencies.py`:
- Around line 432-434: Strengthen the assertions in the dependency hook test
around the returned results so they verify a setuptools requirement excludes
version 82, rather than only checking that setuptools is present. Preserve the
existing requirements argument assertion and ensure the new check would fail if
the hook discarded the returned setuptools constraint.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 4fdc1ee4-6f73-4d81-b707-e30713142a59

📥 Commits

Reviewing files that changed from the base of the PR and between d249228 and 2ffbe2d.

📒 Files selected for processing (4)

• src/fromager/dependencies.py
• src/fromager/hooks.py
• tests/test_dependencies.py
• tests/test_hooks.py

[coderabbitai]

🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Assert that the hook’s returned constraint affects the result.

"setuptools" is already present before the hook runs, so this passes even if setuptools<82 is discarded. Assert that at least one returned setuptools requirement excludes version 82.

Suggested assertion

     names = set(r.name for r in results)
     assert "setuptools" in names
+    assert any(
+        r.name == "setuptools" and Version("82") not in r.specifier
+        for r in results
+    )
     assert called_with["requirements"] == ["setuptools>=40.0"]

As per path instructions, “Verify test actually tests the intended behavior.”

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	names = set(r.name for r in results)
	assert "setuptools" in names
	assert called_with["requirements"] == ["setuptools>=40.0"]
	names = set(r.name for r in results)
	assert "setuptools" in names
	assert any(
	r.name == "setuptools" and Version("82") not in r.specifier
	for r in results
	)
	assert called_with["requirements"] == ["setuptools>=40.0"]

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@tests/test_dependencies.py` around lines 432 - 434, Strengthen the assertions
in the dependency hook test around the returned results so they verify a
setuptools requirement excludes version 82, rather than only checking that
setuptools is present. Preserve the existing requirements argument assertion and
ensure the new check would fail if the hook discarded the returned setuptools
constraint.

Source: Path instructions