Shift folly:string out of folly:base
Shift folly:string out of folly:base

Summary:
```
foundation/dependency_management/ensure-explicit-dependencies.sh folly:string
```

Reviewed By: yfeldblum

Differential Revision: D6411357

fbshipit-source-id: 97afac97e8c1bfb15af5a53fadfb8d6a098a0d37

kwlzn/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,kwlzn/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,wez/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,nodakai/watchman,wez/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,facebook/watchman,kwlzn/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,facebook/watchman,wez/watchman,facebook/watchman,kwlzn/watchman,wez/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,wez/watchman