Support --pip overrides via PEX deps.

[jsirois]

Now, when creating a venv using pex tools and the --pip option from a
PEX that includes pip or setuptools or both, those embedded versions
will be used in place of the conflicting versions installed via --pip
if --collisions-ok is also specified; otherwise the venv pex tool will
error.

Closes #2105

[cognifloyd]

This looks great to me. Thanks!