use ImportWarning for missing package __init__
use ImportWarning for missing package __init__

If a script is located in a directory without an __init__,
a warning was printed on stderr.
Script creators complained about this on pywikipedia-l, as
stderr can be used to contain data to be piped to other
processes.

Also add main().

Change-Id: Ia4aa8dab7203fa1498c0684759641d6ef7c8e139

hasteur/g13bot_tools_new,darthbhyrava/pywikibot-local,valhallasw/pywikibot-core,xZise/pywikibot-core,VcamX/pywikibot-core,jayvdb/pywikibot-core,npdoty/pywikibot,npdoty/pywikibot,magul/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,smalyshev/pywikibot-core,emijrp/pywikibot-core,trishnaguha/pywikibot-core,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,icyflame/batman,happy5214/pywikibot-core,jayvdb/pywikibot-core,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,TridevGuha/pywikibot-core,Darkdadaah/pywikibot-core,h4ck3rm1k3/pywikibot-core,magul/pywikibot-core