do not read biglib/arch/.edmplugincache files for duplicate pluins

This should fix the following duplicates
https://cmssdt.cern.ch/SDT/cgi-bin/showDupDict.py//slc6_amd64_gcc481/www/thu/7.2-thu-06/CMSSW_7_2_X_2014-07-17-0600/testLogs/dupDict-edmPD.log