Switch relay to clap v3

Summary:
Changes references from structopt to clap v3 in the relay compiler code. Code updated by running, while in the root directory for relay-compiler:

```
fastmod -F structopt clap .
fastmod -F clap::clap clap .
fastmod -F clap::StructOpt clap::Parser .
fastmod -F 'derive(StructOpt' 'derive(Parser' .
```

Reviewed By: alunyov

Differential Revision: D34247721

fbshipit-source-id: a98f890c689e8ffd4d751ef75219313354d74325

compiler/crates/relay-compiler/src/main.rs

@@ -53,7 +53,7 @@ struct Opt {
 
     /// Looks for pending changes and exits with non-zero code instead of
     /// writing to disk
-    #[structopt(long)]
+    #[clap(long)]
     validate: bool,
 }