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

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.

Also add some sorting to maintain deterministic order when running
in python3.

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.

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.

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".