vendor: replace vndr with go mod vendor

- use `vendor.mod` instead of `go.mod` to avoid issues to do with
  use of CalVer, not SemVer
- ensure most of the dependency versions do not change
  - only zookeeper client has to change (via docker/libkv#218) as
    previously used version is no longer maintained and has missing
    dependencies

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.gitignore

@@ -21,3 +21,8 @@ vendor/pkg/
 go-test-report.json
 profile.out
 junit-report.xml
+
+# top-level go.mod is not meant to be checked in
+/go.mod
+# workaround go.mod for autogen/winresources/dockerd
+/hack/make/.resources-windows/go.mod