rustPlatform.importCargoLock: add an assert for old Cargo.locks

near the end of 2019, the default Cargo.lock format was changed to
[[package]]
checksum = ...

This is what importCargoLock assumes. If the crate had not been `cargo
update`'d with a more recent toolchain than the one with the new
format as default, importCargoLock would fail when trying to access
pkg.checksum.

I ran into such a case (shamefully, in my own crate) and it took me a
while to figure out what was going on, so here is an assert with a
more user friendly message and a hint.

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