Support environments where pip is not installed

[sbidoul]

This could be done by providing pip-deepfreeze with a target pip command in addition to a target python interpreter.
That pip command could then be target-pip --python {target-python} ....

[sbidoul]

Or, we could install pip as a dependency of pip-deepfreeze, and use {sys.executable} -m pip --python {python} install.

We need to check that the pip version we have is compatible with the target python, though.