Ignore Lib_skip during init
Ignore Lib_skip during init

Summary:
This case was added in D3148666 (https://github.com/facebook/flow/commit/6c0c50983fe0348fab8adfcf05f9fb3fab3a5c1d), which also includes a comment suggesting that
this case should never happen, which appears to be true.

In the stacked diff, Lib_skip becomes possible, specifically in the case where a
lib file is included twice.

This diff changes the behavior of Lib_skip to stop returning `lib_ok=false` when
this case is hit. When this flag is false, Flow does not check files during a
full init -- see check in src/server/server.ml.

Reviewed By: mroch

Differential Revision: D35751393

fbshipit-source-id: 7ae20046a2d759425da879879e95d6db1bbb96a7

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