Improve performance of case-sensitive file key checking

Our Maven site now has about 150,000 Doxia files to render (mostly Markdown and some ASCII doc). During profiling, I realized the Maven site plugin was spending around 18 minutes in the addModuleFiles() method. When looking at the code, I realized the algorithm was *O(n² - n / 2)* the way it adds each file it discovers to a map and the iterates through the map for each file.