Warn against importing types from other libdefs.
Warn against importing types from other libdefs.

Based on discussion in [#1857](https://github.com/flowtype/flow-typed/issues/1857).
doberkofler/flow-typed,mwalkerwells/flow-typed,orlandoc01/flow-typed,flowtype/flow-typed,splodingsocks/FlowTyped,flowtype/flow-typed,mwalkerwells/flow-typed,orlandoc01/flow-typed,splodingsocks/FlowTyped,doberkofler/flow-typed