* pip - Use pip from the current Python interpreter.
If `executable` and `virtualenv` were not specified, and
the `pip` Python module is available for the current interpreter,
use that `pip` module instead of searching for a `pip` command.
* Add comment about needing `__main__` to run `pip`.
* Fix unit test.
* Add porting guide entry.
* Update changelog to match porting guide description.
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