using object that have the same name as submodules as the weird effect
of makeing `import pip.commands.<something> as <anothername>` fail with
a key error. This fixes it by renamin commands as command_dict and fixin
a few imports to accomodate.
Related to #2149
2) have `create_main_parser` mark the parser as the 'main' parser.
3) remove an extraneus block that's never called. thanks @netspyer.
4) move 'create_main_parser` to __init__ to avoid circular or embedded imports.
* Move virtualenv creation out of TestPipEnvironment
* Remove global state and force explicit use of TestPipEnvironment
instances
* Remove "backup" virtualenv copying and instead create new
virtual environments each time.
* Remove the monkeypatched "PyPICache" functionality
* Remove things that were not being used anymore and were dead
weight
* Remove sitecustomize support which was primarily used to
monkeypatch the "PyPICache" but was used in one or two other
tests.
* Better output with the bare asserts we use throughout the tests
* Function fixtures are pretty nice, especially as a way to
start a background server or create an isolated virtualenv
