pwb.py: change path ordering to prioritize own externals
pwb.py: change path ordering to prioritize own externals

After this change, our bundled httplib2 will have priority
over any globally-installed version.

Change-Id: I51b018cd428e922cbccda1274d44aaf5232d07fe

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