multifeed/aggregator: add-only run of IWYU on dependents (3/5) (5/10)
multifeed/aggregator: add-only run of IWYU on dependents (3/5) (5/10)

Summary:
Run IWYU (https://fburl.com/fbcode-iwyu) in "add-only" mode on all transitive
dependents of multifeed sources used in multifeed/aggregator builds:

```
$ buck query 'kind(cxx_.*, rdeps(set(//admarket/... //insights/... //multifeed/...), kind(cxx_.*,deps(fbcode//multifeed/aggregator:aggregator_link) intersect set(//multifeed/...))))'  | sort > /var/tmp/targets.txt
$ buck run tools/build/iwyu:iwyu -- -j 24 --add-only $(split -n l/3/5 /var/tmp/targets.txt)
```

Differential Revision: D18856262

fbshipit-source-id: 2d4aeb07a7eb99693f10d0a38db2aaacda86f1d6

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq