Remove implicit dependency on buildifier

[ian-h-chamberlain]

Only loading com_apt_itude_rules_pip and calling pip_repository rules, e.g.

load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
load("@com_apt_itude_rules_pip//rules:repository.bzl", "pip_repository")
load("//python:versions.bzl", "PYTHON2", "PYTHON3")

maybe(
    pip_repository,
    name = "pip2",
    python_interpreter = PYTHON2,
    requirements_per_platform = {
        "//thirdparty/pip/2:requirements-linux.txt": "linux",
        "//thirdparty/pip/2:requirements-osx.txt": "osx",
    },
)

maybe(
    pip_repository,
    name = "pip3",
    python_interpreter = PYTHON3,
    requirements_per_platform = {
        "//thirdparty/pip/3:requirements-linux.txt": "linux",
        "//thirdparty/pip/3:requirements-osx.txt": "osx",
    },
)

Results in the following error:

ERROR: Analysis of target '//thirdparty/pip/3:compile' failed; build aborted: error loading package '@com_apt_itude_rules_pip//': Unable to find pa
ckage for @com_github_bazelbuild_buildtools//buildifier:def.bzl: The repository '@com_github_bazelbuild_buildtools' could not be resolved.
INFO: Elapsed time: 0.233s

The problem seems to be caused by https://github.com/apt-itude/rules_pip/blob/master/BUILD where buildifier is being loaded. This should be moved to a private BUILD file instead of being publicly visible in the top level, so that other libraries depending on rules_pip are not required to depend on buildifier (and therefore go toolchains, etc).