Use one Python virtual environment for all mach commands

Prior to this commit:

* Our Python dependency story was a bit of a mess. We had complete
 Python packages (wheels and directories) living in-tree, despite
 not having any changes from upstream. This is particularly bad because
 `setup.py` never gets run on these packages which could (sometimes
 silently) unintended breakage.
* Python virtual environments (virtualenv) were only utilized for
 testing web-platform tests

After this commit:

* A single virtualenv (`python/_virtualenv`) is activated upon *every*
 call to mach
* A requirements file (`python/requirements.txt`) is added to describe
 the dependencies needed by Python modules in `python/`. The child
 commit immediately following this will remove all the dependencies
 no longer needed in-tree (for the sake of keeping this commit
 readable).

Relevant to https://github.com/servo/servo/issues/861

Fixes https://github.com/servo/servo/issues/6999

python/requirements.txt

@@ -0,0 +1,12 @@
+# 'mach' is not listed here because a new version hasn't been published to PyPi in a while
+
+blessings == 1.6
+mozdebug == 0.1
+mozinfo == 0.8
+mozlog == 3.0
+toml == 0.9.1
+
+# For Python linting
+flake8 == 2.4.1
+pep8 == 1.5.7
+pyflakes == 0.8.0