Add some option to the generator to help it search for files with graphql tags

[mwildehahn]

From @jjergus:

Probably the #1 thing to address for now is to make sure the build time is O(number of GraphQL types), not O(your whole codebase)

https://hacklang.slack.com/archives/C020EEF5EQN/p1620846601088900?thread_ts=1620846190.086500&cid=C020EEF5EQN

[jjergus]

There is https://docs.hhvm.com/hack/reference/function/HH.Facts.types_with_attribute/ but AFAIK it requires ext_facts which may need a very recent HHVM version.