[src-expose] Add flag for not recursively serving repos

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/768958891

See discussion starting at https://sourcegraph.slack.com/archives/CTJCMDCCS/p1593043754316900?thread_ts=1593010018.312200&cid=CTJCMDCCS for context

Short story: they have a lot of large monorepos, which are causing our dockerized src-expose to fail K8s health checks in their deployment.

As far as I can tell this fix doesn't actually work 🤦 but I've never built Go CLIs before... I just get the error:

flag provided but not defined: -recurse

Can either of you help take this on? Or let me know if you believe something else is the issue here.