With pip3.7, `--index-url ""` is different from absence of --index-url
argument.  Apply the same for python3 variant

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

This guarantees for the package feeds that
the mk files will always be available for all packages.

Will need to see about external-feed Python packages
a bit later.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

I admit this may be be a bit aggressive, but the lang
folder is getting cluttered/filled up with Python, PHP, Perl,
Ruby, etc. packages.

Makes sense to try to group them into per-lang folders.

I took the Pythons.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>