Support `cargo clean`

[ehuss]

The cargo clean command currently does not know about std dependencies. There are multiple issues here:

• -p for a user crate doesn't work because the hash is incorrect.
• -p for a standard library doesn't work because it doesn't know about these dependencies.

I'm inclined to rearchitect how clean works instead of trying to shoehorn these fixes, since there are a number of other problems with how clean works.

[ehuss]

Triage: cargo clean -p <user crate> now works since cargo clean was redesigned in rust-lang/cargo#8210.

-p for standard library dependencies still doesn't work.