Fix bug when cargo-contract installation had been interrupted

[cmichi]

Fixes #513.

If the crate installation had been interrupted the file _Cargo.toml had already been renamed to Cargo.toml. If the installation was resumed the _Cargo.toml was no longer found. This resulted in the panic Failed renaming _Cargo.toml to Cargo.toml: File not found..

I fixed it by copy (with overwrite) the file _Cargo.toml to Cargo.toml instead.

[athei]

Without suggesting that you should do that here (but it wouldn't be too hard): A more elegant solution would use a guard type that does the cleanup on Drop. This will be safe against panics. The current code leaves the Cargo.toml in the directory on panic. Not optimal.

[athei]

This is fine. But you could be a total champ and put the type into a private sub module and do the copy/rename in the constructor in order to have proper RAII. We don't need the copy and stick with rename in this case even.