title: Bazel build system: how to reference BUILD file for external http_archive in workspace?
body: <p>I'm currently trying to update some build files from Bazel 0.19 to Bazel 3.2.0</p>
  
  <p>I have this folder structure</p>
  
  <pre><code>Projectfolder/
      |-WORKSPACE
      |-third_party/
          |-openexr.BUILD
  </code></pre>
  
  <p>previously, I had the following defined in my WORKSPACE file:</p>
  
  <pre><code>...
  
  new_http_archive(
      name = "openexr",
      build_file = "third_party/openexr.BUILD",
      strip_prefix = "openexr-2.2.0",
      urls = ["https://github.com/openexr/openexr/archive/v2.2.0.zip"],
  )
  ...
  </code></pre>
  
  <p>new_http_archive and http_archive got merged into one in new bazel, but I'm not sure what the correct syntax is now. the following</p>
  
  <pre><code>load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
  
  http_archive(
      name = "openexr",
      build_file = "@//third_party:openexr.BUILD",
      strip_prefix = "openexr-2.2.0",
      urls = ["https://github.com/openexr/openexr/archive/v2.2.0.zip"],
  )
  </code></pre>
  
  <p>gives me the following error:</p>
  
  <pre><code>Unable to load package for //third_party:openexr.BUILD: BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
  </code></pre>
  
  <p>other syntax that I tried for build_file are:</p>
  
  <pre><code>build_file = "third_party/openexr.BUILD",
  build_file = "third_party:openexr.BUILD",
  build_file = //third_party/openexr.BUILD",
  build_file = //third_party:openexr.BUILD",
  </code></pre>
  
  <p>but all of them give me some errors.</p>
  
  <p>What's the proper syntax compatible with latest (3.2.0) Bazel?</p>
  
  <p>//edit: doing this on Windows</p>