Cargo build plans

[est31]

cargo-udeps could maybe use cargo build plans instead of using cargo as a library in order to generate the save-analysis info it needs. This means we could re-run build plan generation each time and have better rerun capabilities (#5) and in a later step could even get rid of depending on the cargo library (#13).

Some links to upstream issues:

• Build plan support for check/test/doc builds rust-lang/cargo#5855 . Needed.
• [Feature Request] Support cargo build --build-plan oli-obk/cargo_metadata#46 . Technically not needed, but would be nice to have.

[est31]

Due to removal of build plans, this issue becomes irrelevant: rust-lang/cargo#7902