title: Get WORKSPACE root in Bazel BUILD
body: <p>I'm using <a href="https://github.com/pubref/rules_protobuf" rel="nofollow noreferrer">rules_protobuf</a> to build Python language bindings for my <code>helloworld.proto</code> file.  My <code>helloworld.proto</code> imports <a href="https://github.com/google/protobuf/blob/master/src/google/protobuf/wrappers.proto" rel="nofollow noreferrer">wrappers.proto</a>.</p>
  
  <pre><code>syntax = "proto3";
  
  package main;
  
  import "google/protobuf/wrappers.proto";
  </code></pre>
  
  <p>My BUILD file</p>
  
  <pre><code>load("@org_pubref_rules_protobuf//python:rules.bzl", "py_proto_compile")
  
  # Wrapper around proto_compile.
  # https://github.com/pubref/rules_protobuf/blob/master/protobuf/internal/proto_compile.bzl
  
  py_proto_compile(
      name = "py",
      with_grpc = True,
      protos = ["helloworld.proto"],
      imports = ["/usr/local/home/username/myproject/include"]
  )
  </code></pre>
  
  <p>The <code>wrappers.proto</code> file is located in the directory</p>
  
  <pre><code>/usr/local/home/username/myproject/include
  </code></pre>
  
  <p>The Bazel rule <code>py_proto_compile</code> is defined by <a href="https://github.com/pubref/rules_protobuf" rel="nofollow noreferrer">rules_protobuf</a> and <a href="https://github.com/pubref/rules_protobuf/tree/master/protobuf#proto_compile" rel="nofollow noreferrer">documented in the README.md</a>. <code>imports</code> is defined as:</p>
  
  <ul>
  <li><strong>Name:</strong> <code>imports</code></li>
  <li><strong>Type:</strong> string_list`</li>
  <li><strong>Description:</strong> Optional paths to be passed as -I arguments to the protoc tool.</li>
  <li><strong>Default:</strong> <code>[]</code></li>
  </ul>
  
  <p>My BUILD rules works, however I've hard-coded the location of of <code>wrappers.proto</code> with:</p>
  
  <p><code>imports = ["/usr/local/home/username/myproject/include"]</code></p>
  
  <p>Bazel doesn't appear to have any <a href="https://docs.bazel.build/versions/master/be/make-variables.html#predefined_variables" rel="nofollow noreferrer">predefined Make variables</a> that reference my WORKSPACE root. Ideally, I'd like to do something like this:</p>
  
  <p><code>imports = ["$WORKSPACE_ROOT"/include"]</code></p>