[get-def] don't call covers_target on every expression
[get-def] don't call covers_target on every expression

Summary: instead of checking every expression's loc and filtering matching ones if they are `require()` calls, filter `require()` calls and then see if they match the loc

Reviewed By: vrama628

Differential Revision: D18201197

fbshipit-source-id: 12b2b1abc9199e57e1eb022ca8ed3d8799f660cc

mroch/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,samwgoldman/flow,facebook/flow,facebook/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,mroch/flow,mroch/flow,samwgoldman/flow,mroch/flow