…into qgis.processing

Instead of encouraging use of the internal Processing modules
(e.g. from processing.tools.general import run , import processing, ...)
instead expose all Python specific STABLE processing additions
to the qgis.processing module.

Instead, scripts and plugins should now use

from qgis.processing import run, algorithmHelp,...

This makes a clear distinction between internal Processing python
modules (i.e., everything else!) and the parts of Processing
which are stable and designed to be used by plugins and scripts.

TODO: QGIS 4.0 -- move the internal Processing plugin modules
to __processing, to clearer indicate that this is all internal
stuff.