Python Packages: Allow installing extra packages before `pip install -e .`

Closes #229 (closed)

Contains !401 (merged)

The new syntax can be seen in the test module preinstall.json

  "prepare": {
    "python": {
      "packages": {
        "preinstall_test": {
          "preinstall": [
            "packaging"
          ]
        }
      }
    }
  }