This was created in a few steps. "go" in the following is go 1.11-rc1

- Use "go mod init" to migrate the vendor/vendor.json references into
  a new go.mod.

- rm -rf vendor, since we're going to reconstruct that again in a moment.

- "go mod tidy" as a way to get all the necessary stuff installed and the
  go.mod updated for what was actually resolved.

- "go mod vendor" copied all of the dependencies from the module cache
  into the "vendor" directory, and created the `vendor/modules.txt` index
  file.

This can then be built using "go install -mod=vendor .", where that extra
flag forces it to read the dependencies only from the vendor dir and not
from upstream or the local cache.

This is not intended to be merged: it was just an experiment to test out
Go 1.11-rc1 and get a sense of what a transition here might look like.