refactor(paroxysm): Explicitly macro imports, no more 'extern crate'

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>

fun/paroxysm/src/cfg.rs

@@ -1,4 +1,5 @@
 use std::collections::HashSet;
+use serde_derive::Deserialize; // TODO(tazjin): move away from serde_derive
 
 #[derive(Deserialize)]
 pub struct Config {