title: Homebrew formula: use dependency from a custom tap
body: <p>Say I have an app which I'd like to distribute via a custom homebrew tap so that any potential user of the app could install it via</p>
  
  <pre><code>brew tap &lt;github_account&gt;/&lt;appname&gt;
  brew install &lt;appname&gt;
  </code></pre>
  
  <p>My app has a dependency on some thirdparty library. It is not present in the official homebrew repository but it is present in <em>another</em> custom homebrew tap. So while I could just include the library in my own tap and have it a dependency for my app in the formula, it feels the right thing to do to reuse the library's packaging from the existing tap. So my question is: is it possible to specify a dependency from a custom tap within the homebrew formula?</p>