…ditable wheels

…info dir

…r resolves

…field

Installing the editable wheels via a pex was not effective.
Using pex-tools to install two different pexes in the same venv
caused conflicts with the __main__.py file. Using pex_path did
not work because the editable wheel pex only had the editable
wheels, not any of the dependencies. So, installing them in
a pex was not effective.

However, it does work to tell pex that the wheels are "sources".
That way, pex just dumps them in the lib directory. Then, it
was just a matter of adding the post-processing commands to
move, install, and wrap up installation of the editable wheels.

Even though we use pip to install the editable wheels, pants is
both backend and frontend for PEP 660, because we generate the
wheel (delegating dist-info generation to the external backend)
and then install the wheel (delegating unpacking the wheel to
pip). The final piece of complying with PEP-660 is to mark the
install as an editable install via direct_url.json. We generate
that when building the wheel, and then move it into place after
pip has installed the editable wheels for us.

One other method I considered was installing the editable
wheels in the requirements pex. But I could not figure out how
to combine EntireLockfile with additional wheel requirements.
So, I punted and went merely passing the wheels as is via pex
instead of attempting to install them in any pex.

Finally, all of the export and local_dists_pep660 test pass!

This way people have to opt into including the editables (and therefore
incurring the cost of finding/building/installing the PEP-660 wheels).