Split manifest section readers

[codeforester]

Fixes #1461

Summary

• move build manifest parsing into base_setup.manifest_build
• move release manifest parsing into base_setup.manifest_release
• share optional command-runner validation through base_setup.manifest_reader_common
• keep read_manifest() and compatibility model imports on base_setup.manifest stable

Validation

• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_setup/tests/test_manifest_structure.py -q
• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_setup/tests/test_manifest.py -q
• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_setup/tests cli/python/base_release/tests cli/python/base_projects/tests/test_build_targets.py -q
• /private/tmp/base-python-train-venv/bin/pylint cli/python/base_setup/manifest.py cli/python/base_setup/manifest_build.py cli/python/base_setup/manifest_release.py cli/python/base_setup/manifest_reader_common.py cli/python/base_setup/tests/test_manifest_structure.py
• /private/tmp/base-python-train-venv/bin/python -m compileall cli/python/base_setup cli/python/base_release cli/python/base_projects
• git diff --check
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1461-release-smoke bin/basectl release plan --version 1.6.1 --manifest base_manifest.yaml
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1461-build-smoke bin/basectl build base-demo --workspace /Users/rameshhp/work --list