STYLE: Add icon for Extension Wizard module

Reorganize Extension Wizard module resources to account for the
directory structure being conducive to generating a compiled resource
script in a reasonable location. Add rules to generate and import the
same. Add code to set the module icon resource.
COMP: Ensure output directory exists

Tweak slicerFunctionAddPythonQtResources to also include a rule to
ensure that the output directory for the resource script exists. This
fixes build errors on non-Ninja clean builds.
ENH: Also find templates in build (source) tree

Add slicer.util.sourceDir() convenience method to retrieve the Slicer
source directory, when running from a local build tree. Change Extension
Wizard module to prefer to use templates from the same, if available.