Removes all but one occurence of `extern crate`, to conform with Rust
2018.
The last one is necessary because Diesel is a giant jungle of
complicated macros re-exported from private crates, and the current
version makes it hard to import those directly instead.
Change-Id: Id14165a456d5c3ce6f7a4e0222c611640113eb11
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1549
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>
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