title: Bazel: relative local path as url in http_archive()
body: <p>I am trying to include an external library into my Bazel project.
  It is a commercial, closed source software library that comes as a bunch of <code>.h</code> and <code>.a</code> files in a tar file (Linux). There is no public download link, you have to download the archive manually somewhere.</p>
  
  <p>Thus, I checked the library archives into Git (let's not discuss this here) in <code>vendor/</code> and use</p>
  
  <pre class="lang-py prettyprint-override"><code>http_archive(
      name = "library_name",
      build_file = "//third_party:library_name.BUILD",
      sha256 = "12165fbcbac............",
      urls = ["file:///home/myuser/git/repo/vendor/libraryname.tar.gz"],
  )
  </code></pre>
  
  <p>I would like to not use an absolute path in <code>urls=</code>, so my coworkers can checkout and build the library without hassle. How can I use a relative path with <code>http_archive()</code>?</p>
  
  <p>I have looked at <a href="https://stackoverflow.com/questions/48545575/how-to-get-workspace-directory-in-bazel-rule">this answer</a>, but it seems to be not exactly the same problem and the example is incomplete.</p>