namespace leap + leap.common split

leap is a namespace package from here on.
common folder will be deleted and moved to leap_pycommon repository.
remove common module!

from now on this will live in a common repo,
leap_common.

We still need to add it to requirements
add external leap.common to requirements

and modify parse_requirements to skip it for now.
add changes file
remove branding remnant, add missing header
move abc import to its place