Implement `preserve_egg_dir`

[wolfv]

Was preserve_egg_dir in conda-build.

Preserve the egg dir for Python packages.

This is needed for some packages that use features specific to setuptools. The default is False.

Used a couple of times on conda-forge: https://github.com/search?q=org%3Aconda-forge+preserve_egg_dir+language%3AYAML&type=code

build:
  python:
    # used on conda-forge, still needed?
    preserve_egg_dir: bool (default false)