title: go build can't find a revision
body: <p>I developed a repo on computer A and created a go.mod/go.sum that I checked in.</p>
  
  <p>I pull that repo with the go.mod/go.sum files on computer B, but when I try to build the program, the module constraints can't be satisfied.</p>
  
  <pre><code>$ go build
  go: finding github.ibm.com/kms/key-protect-client v0.1.5
  go: finding golang.org/x/tools v0.0.0-20180221164845-07fd8470d635
  go: github.ibm.com/kms/key-protect-client@v0.1.5: unknown revision v0.1.5
  go: error loading module requirements
  </code></pre>
  
  <p>The repo that is failing is a private repo, and for some reason it doesn't get downloaded to the module cache. On another computer, the dependencies are downloaded and the build succeeds. I am building another private repo in that same domain, so I know that my github credentials give me access to these repos. But for some reason, the go module system can't get to the dependent repo.</p>
  
  <p>I cannot find more information how to debug this.</p>