[BUGFIX] "partialRootPath" in a "registerFluidFlexFormPlugin" won't work, if "EXT:" prefix is used

This change modify the method "setTemplatePaths" in the class Tx_Flux_Provider_Configuration_Fallback_ConfigurationProvider
which use the function "t3lib_div::getFileAbsFileName" if the prefix "EXT:" is used.

Fixes: #196