title: enforce package order when installing with pip using a wheel file
body: <p>I have a python <code>requirements.txt</code> file as follows:</p>
  
  <pre><code>torch==1.0.0
  torchvision==0.2.1
  opencv_python==3.4.2.17
  numpy==1.15.1
  gputil==1.3.0
  scikit_learn==0.19.2
  scikit_image==0.14.0
  sk_video==1.1.10
  git+https://github.com/facebookresearch/maskrcnn-benchmark.git@5ec0b91cc85163ac3b58265b3f9b39bb327d0ba6
  </code></pre>
  
  <p>Firstly, I generate a wheel file as follows:</p>
  
  <pre><code>pip wheel --wheel-dir=wheels --no-deps .
  </code></pre>
  
  <p>Then I install it as:</p>
  
  <pre><code>pip install --no-cache -r requirements.txt --find-links=./wheels
  </code></pre>
  
  <p>However, no matter where I put the <code>git</code> project in my <code>requirements.txt</code> file, it is always the first thing <code>pip</code> tries to install. This is problematic as the other packages need to be installed first. How can I specify or enforce some order in the package installation?</p>