Use bottled inputs and add RuboCop guard

[MikeMcQuaid]

• Discover CPython wheels already packaged in each keg.
• Expand PyPy bootstrap archives packaged in libexec.
• Resolve dependency paths without loading formula source.
• Enforce source independence with RuboCop and API-loaded tests.
• Keep the internal JSON API unchanged and minimal.

Fixes #23465

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OpenAI Codex 5.6 Sol xhigh with local review and testing.

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[Copilot]

Pull request overview

This PR tightens Homebrew’s structured install-step runners to avoid depending on formula source and post-pour resources by switching to bottled inputs and API-derived context, and it adds enforcement via a dedicated RuboCop cop plus accompanying specs.

Changes:

• Add a new Homebrew/InstallStepsSourceIndependence RuboCop cop and spec coverage to prevent formula/source/resource lookups and direct downloads in install-step runners.
• Update install-step execution logic to resolve helper-tool paths and pkgetc paths without loading formula source.
• Update install-step specs to use API-loaded formula structs and to validate bootstrapping behavior against bottled wheel/archive inputs.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Library/Homebrew/test/rubocops/install_steps_source_independence_spec.rb	Adds spec coverage for the new RuboCop cop’s allowed/forbidden patterns.
Library/Homebrew/test/install_steps_spec.rb	Updates tests to use API-loaded formula structs and validates bootstrapping via bottled wheel/archive inputs and opt-path tool resolution.
Library/Homebrew/rubocops/install_steps_source_independence.rb	Introduces the RuboCop cop enforcing install-step source independence.
Library/Homebrew/rubocops/all.rb	Registers the new RuboCop cop for loading.
Library/Homebrew/install_steps/formula_actions.rb	Switches CPython/PyPy bootstrapping to use bottled wheels/archives rather than formula resources.
Library/Homebrew/install_steps.rb	Resolves helper-tool and pkgetc paths without loading formula source.
Library/.rubocop.yml	Enables/scopes the new cop to install-step runner files.
docs/Formula-Cookbook.md	Documents that named actions must use bottled inputs rather than formula resources after pouring.

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