Revert of Removed virtualenv from depot_tools

bootstrap/virtualenv/docs/installation.rst

Index: bootstrap/virtualenv/docs/installation.rst
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+Installation
+============
+
+.. warning::
+
+    We advise installing virtualenv-1.9 or greater. Prior to version 1.9, the
+    pip included in virtualenv did not download from PyPI over SSL.
+
+.. warning::
+
+    When using pip to install virtualenv, we advise using pip 1.3 or greater.
+    Prior to version 1.3, pip did not download from PyPI over SSL.
+
+.. warning::
+
+    We advise against using easy_install to install virtualenv when using
+    setuptools < 0.9.7, because easy_install didn't download from PyPI over SSL
+    and was broken in some subtle ways.
+
+To install globally with `pip` (if you have pip 1.3 or greater installed globally):
+
+::
+
+ $ [sudo] pip install virtualenv
+
+Or to get the latest unreleased dev version:
+
+::
+
+ $ [sudo] pip install https://github.com/pypa/virtualenv/tarball/develop
+
+
+To install version X.X globally from source:
+
+::
+
+ $ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz
+ $ tar xvfz virtualenv-X.X.tar.gz
+ $ cd virtualenv-X.X
+ $ [sudo] python setup.py install
+
+
+To *use* locally from source:
+
+::
+
+ $ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz
+ $ tar xvfz virtualenv-X.X.tar.gz
+ $ cd virtualenv-X.X
+ $ python virtualenv.py myVE
+
+.. note::
+
+    The ``virtualenv.py`` script is *not* supported if run without the
+    necessary pip/setuptools/virtualenv distributions available locally. All
+    of the installation methods above include a ``virtualenv_support``
+    directory alongside ``virtualenv.py`` which contains a complete set of
+    pip and setuptools distributions, and so are fully supported.