Improve revision detection in DeduceConstraint

[carolynvs]

DeduceConstraint currently uses a simple check to determine if a string is a revision. But we can do better!

By moving this into *sourceMgr, we create an opportunity to do better validation here. Rather than deducing whether it's a revision from what general rules we have about the patterns of the bytes, we can actually hand this off to a sourceGateway, which can validate that it's an appropriate revision for that source type.

https://github.com/golang/dep/pull/761/files#r122593455

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This is for after #761 is merged, which moves DeduceConstraint from the ensure command into SourceManager. Also note that #715 also touches the function, so waiting for that to settle down would be best.

[carolynvs]

Closing this in favor of #987