ENH: Skip loading of loadable python module if loadable module are disabled

Before this commit even after disabling the loading of loadable module
the python module corresponding to "logic", "Widgets, "displayable
manager" and "mrml" where still imported. This commit prevent this.

Note that if a given loadable module is disabled with the application
settings the associated python modules will still be imported.

Ideally python module should be imported on a per module basis as it is
done for extension. This will be possible only after python module specific
to a loadable module are located in dedicated directory. This is
somewhat related to issue 0001365

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22463 3bd1e089-480b-0410-8dfb-8563597acbee