This CL introduces package_management.py, a module for managing third-party Python packages in a site directory within depot_tools. A future CL will integrate this with depot_tools, causing the packages to be downloaded and installed on first run after that CL lands. Once in place presubmit_canned_checks will be extended to make use of the locally installed version of pylint.

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Review URL: http://codereview.chromium.org/8965033

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@120225 0039d316-1c4b-4281-b951-d872f2087c98