[v2] Generate dependency lockfiles

[hssyoo]

To provide a stable build environment, this PR does the following:

• Move requirements.txt -> requirements-dev.txt to align with AWS CLI v1 and boto projects
• Alias requirements.txt to requirements-dev.txt for backwards compatibility
• Create a requirements-build-win.txt file as an extra source for pip-compile so lockfiles include Windows dependencies
• Move from mock to unittest.mock import to remove the dependency and simplify dependency resolution
• Generate lockfiles from requirements*.txt files and replace their references in install scripts with requirements*-lock.txt