Add third-party dependency detection and inclusion for subfolder builds

• Extract third-party dependencies from all Python files (including copied external dependencies)
• Add dependencies to temporary pyproject.toml's [project.dependencies] section
• Handle cases where dependencies aren't installed in build environment by treating ambiguous imports as third-party
• Add test to verify third-party dependencies are correctly added
• Fix test cleanup assertion to account for pre-existing files

Full Changelog: v1.4.1...v2.0.4