imports: try to use global imports unless it is a layering violation

To minimize the impact, do imports that violate the layering at runtime instead
of at import time.

py3: trivial renaming of .iteritems() to .items()

A bit like "2to3 -f dict", but we don't want list().

py3: add b'' annotations in some places where they will be needed later

Mostly entirely trivial adding of b prefix that is a ignored for py2 ... and
also a bit of related trivial reformatting/refactorings.

vcs: always return bytes from node.content

We will rather have the unicode conversions explicit.

Note: Py3 bytes doesn't have .startswith - replace that with a regexp.

py3: cleanup map usage and avoid py3 ambiguity

Based on 2to3 -f map ... but replace map with something more explicit (unless
born and raised in a lisp world) (but sometimes slightly more verbose).

Second step in two-part process to rename directories.
This is the actual directory rename.