Fix performance regression - network calls too early

[sdboyer]

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Fix the performance regression arising from how early we go out over the network to verify a source's upstream existence: https://github.com/sdboyer/gps/pull/196#issuecomment-289955050

This may be a nontrivial fix, as it would create a situation where it's possible for a source that initially looks good to become bad later.

[fabulous-gopher]

This issue was moved to golang/dep#415