Add editable install support to `pip-install` (#675)
Per the title: adds support for `-e` installs to `puffin pip-install`.
There were some challenges here around threading the editable installs
to the right places. Namely, we want to build _once_, then reuse the
editable installs from the resolution. At present, we were losing the
`editable: true` flag on the `Dist` that came back through the
resolution, so it required some changes to the resolver.
Closes https://github.com/astral-sh/puffin/issues/672.
