Make python interpreter configurable to support python3

[scele]

• Add pip_version_proxy to simplify picking correct pip deps

  The new pip_version_proxy rule generates a requirements.bzl file that provides a requirements() function that resolves to the correct version of python dependencies depending on the current python runtime. This is used via the WORKSPACE pattern:

  pip_import(
      name = "pip_deps2",
      requirements = ["//:requirements-pip.txt"],
      python = "@python2//:bin/python",
  )
  pip_import(
      name = "pip_deps3",
      requirements = ["//:requirements-pip.txt"],
      python = "@python3//:bin/python",
  )
  load("@pip_deps2//:requirements.bzl", pip_install2 = "pip_install")
  pip_install2()
  load("@pip_deps3//:requirements.bzl", pip_install3 = "pip_install")
  pip_install3()
  
  pip_version_proxy(
      name = "pip_deps",
      define = "python",
      values = {
          "py2": "@pip_deps2",
          "py3": "@pip_deps3",
      },
  )

  With this code, requirement("foo") would map to @pip_deps2 when running "bazel build --define python=py2", and to @pip_deps3 when running with "bazel build --define python=py3".

• Make python interpreter configurable

  We need to propagate the python interpreter through all pip_import and wheel rules, to support using two different interpreters (read: python2 and python3) in the same workspace.

• Check that python headers are installed

  Lack of python headers (/usr/include/pythonX/...) on the host machine may affect how wheels are built, which in turn can change the filenames of the wheels. For example, "pip wheel wrapt==1.10.11" produces "wrapt-1.10.11-py3-none-any.whl" if headers are not available, and "wrapt-1.10.11-cp35-cp35m-linux_x86_64.whl" if they are. Since we bake the wheel name into the locked requirements.bzl file, the build would break depending on header availability.

• Fix BUILD file generation for dirty wheels

  When extracting foo_dirty wheel repository, we pass all wheels as an argument to piptool that transitively depend on foo, so that piptool will extract all of them to the foo_dirty repository. After that, piptool generates a BUILD file from the first wheel, expecting that to be foo.whl. This was not happening, since we were passing wheels in random order to piptool extract.

• Generate missing init.py files when extracting wheels

  This allows us to stop relying on legacy_create_init=True attribute.