title: malformed module path "xxxx/xxxx/uuid" missing dot in first path element when migrating from GOPATH based dep to go mod
body: <pre>
  $ go version
  1.13.3
  </pre>
  
  <p>I have a folder structure as follows:</p>
  
  <pre><code>GOPATH
  +---src
       +--- my-api-server
             +--- my-auth-server
                   +--- main.go
             +--- my-utils
                   +--- uuid
                         +--- uuid.go
  </code></pre>
  
  <p><code>my-auth-server</code> uses <code>my-api-server/my-utils/uuid</code> as a depenency</p>
  
  <p>Now, when I used the GOPATH based module system, this worked fine. But when using go modules, when I run <code>go run main.go</code> in <code>my-auth-server</code> it returned error:</p>
  
  <pre><code>build command-line-arguments: cannot load my-api-server/my-utils/uuid: malformed module path "my-api-server/my-utils/uuid": missing dot in first path element
  </code></pre>
  
  <p>Any idea how to solve this?</p>