Godep changes source code it vendors

[twaugh]

When godep vendors source code it removes "import comments" from package statements. We will need to accommodate this somehow.

Perhaps we need to

• identify whether the top-level package using godep (look for Godeps/Godeps.json)
• if so, for each upstream tag/commit in a vendored project, try first with 'git ls-tree ...' and then if that fails try building a FileHashes map from files in the working tree (that match the subset we are looking for) that we modify in the same way as godep