title: File does not reside within any path specified using proto_path
body: <p>I am testing out importing .proto file from another directory.</p>
  
  <p><strong>$GOPATH/src/A/A.proto</strong></p>
  
  <pre><code>syntax = "proto3"; 
  package A;
   message SomeMsg {
       string  msg = 2;
       int64   id  = 3;
   }
  </code></pre>
  
  <p><strong>$GOPATH/src/B/B.proto</strong></p>
  
  <pre><code>syntax = "proto3"; 
  package B; import "A/A.proto";
   message Msg {
       SomeMsg s = 1;
   }
  </code></pre>
  
  <p>I'm doing this:
  in folder A:</p>
  
  <pre><code>protoc A.proto --go_out=.
  </code></pre>
  
  <p>and then in folder B:</p>
  
  <pre><code>protoc B.proto --go_out=. --proto_path=$GOPATH/
  </code></pre>
  
  <p>But I will get this error:</p>
  
  <blockquote>
    <p>B.proto: File does not reside within any path specified using --proto_path (or -I).  You must specify a --proto_path which encompasses this file.  Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think).</p>
  </blockquote>