title: pip install editable working dir to custom path using requirements.txt
body: <p><strong>Short version:</strong>
  Is it possible to use the -e parameter in requirements.txt with a path where the editable package should be installed?</p>
  
  <hr>
  
  <p><strong>First approach</strong></p>
  
  <p>requirements.txt:</p>
  
  <pre><code>-e git+https://github.com/snake-soft/imap-storage.git#egg=imap-storage
  </code></pre>
  
  <p><strong>Pro:</strong> Automated install</p>
  
  <p><strong>Contra:</strong> Editable directory is inside virtualenv src folder (not in workspace)</p>
  
  <hr>
  
  <p><strong>Second approach</strong> <em>(Edit: Don't use this until you know what you're doing, look at bottom)</em></p>
  
  <p>If i clone the repo and installed it like this (virtualenv activated):</p>
  
  <pre><code>cd /home/user/workspace
  git clone https://github.com/snake-soft/imap-storage.git
  pip install -e .
  </code></pre>
  
  <p>Gives the structure i want:</p>
  
  <pre><code>workspace
  ├── imap-storage
  ├── django-project  # uses imap-storage module
  </code></pre>
  
  <p>I have what i want. The repository (imap-storage) lays parallel to the django-project, that uses it.
  It is importable because it is installed inside the virtualenv.</p>
  
  <p><strong>Pro:</strong> Editable directory is inside my workspace</p>
  
  <p><strong>Contra:</strong> Not automated, not intuitive</p>
  
  <hr>
  
  <p><strong>Goal</strong></p>
  
  <ul>
  <li>pip install -r requirements.txt to install module from git (like first approach)</li>
  <li>Module is in pythonpath of virtualenv -> importable</li>
  <li>Editable working dir of the module is in my workspace (like second approach)</li>
  </ul>
  
  <hr>
  
  <p><em>PS: Or am i completely wrong-thinking and should go for something completely different?</em></p>