Refactor relenv.build.common into smaller pieces.

[dwoz]

Modules

init.py

Public API surface. Re-exports 11 key items from submodules for backward compatibility.

builder.py

• Builder class: Core build orchestration, recipe management, dependency ordering
• Dirs class: Working directory structure (root, source, build, prefix, logs)
• builds: Global Builder registry
• get_dependency_version(): Fetch dependency metadata from python-versions.json

builders.py

Specific build functions for dependencies:

• build_openssl() / build_openssl_fips()
• build_sqlite()
• build_default(): Generic configure/make/make install

download.py

• Download class: Manages tarball downloads with URL templating, checksums, fallback URLs
• verify_checksum(): SHA-1/SHA-256 validation

install.py

Post-build installation tasks:

• install_runtime(): Copy runtime files into build
• finalize(): Create symlinks, patch shebangs, fix rpaths
• create_archive(): Package build into tarball
• update_ensurepip(): Install bundled pip
• patch_file() / patch_shebang() / patch_shebangs(): Text/binary patching

platform.py

Platform detection and triplet generation (x86_64-linux-gnu, etc.)

ui.py

Build progress tracking:

• BuildStats: Persistent build timing/line count statistics
• LineCountHandler: Real-time progress estimation using exponential moving averages
• Persistent JSON storage for learning build patterns

Dependencies

Platform-specific modules (linux.py, darwin.py, windows.py) import from this package
and add platform-specific recipes to builds registry.