python3: Split pip into separate source package

Packaging pip from a separate source package allows it to stay updated
with upstream.

Host pip will remain installed as part of python3. Host pip is used in a
much more controlled way and so is less critical for it to track
upstream.

This also removes the python-pip-conf package and installs the pip.conf
file as part of python3-pip.

The patch 003-disable-pip-version-check.patch is originally from Debian:
bb079efb8c/debian/patches/disable-pip-version-check.patch

pip was in a separate source package that was removed in
a53d0c5a403d1669e2cf6c59c2be6a9d3ed633a0; this work is not based on that
earlier package.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>
(cherry picked from commit eee273507b)

lang/python/python-pip/patches/003-disable-pip-version-check.patch

@@ -0,0 +1,20 @@
+From: Barry Warsaw <barry@python.org>
+Date: Fri, 18 Nov 2016 17:07:47 -0500
+Subject: Set --disable-pip-version-check=True by default.
+
+Patch-Name: disable-pip-version-check.patch
+---
+ src/pip/_internal/cli/cmdoptions.py | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/src/pip/_internal/cli/cmdoptions.py
++++ b/src/pip/_internal/cli/cmdoptions.py
+@@ -892,7 +892,7 @@ disable_pip_version_check: Callable[...,
+     "--disable-pip-version-check",
+     dest="disable_pip_version_check",
+     action="store_true",
+-    default=False,
++    default=True,
+     help="Don't periodically check PyPI to determine whether a new version "
+     "of pip is available for download. Implied with --no-index.",
+ )