Add cargo dv clean and make dependency analysis recursive

[rikosellic]

This PR enhances the dependency analysis in several ways:

• Added cargo dv clean to remove Verus-generated libraries without touching regular Rust dependencies. This makes recompilation significantly faster than using cargo clean. You can also run cargo dv clean --targets {name} to clean a specific target.
• The compile and verify commands now recursively detect dependent Verus targets and automatically compile any missing dependencies. As a result, running cargo dv clean --targets vstd_extra followed by cargo dv verify --targets ostd is now fully legal.