Added "LICENSE.txt" and "README.txt" as recognized filenames

The webstack packages (HttpCommon, Meddle, etc) have license files named "LICENSE.txt", which means they don't currently get detected by PackageEvaluator. This fixes that by adding both "LICENSE.txt" and "README.txt" as recognized file names.

This is probably not the best fix, since you might want to accept README.* and LICENSE.*, rather than only specific endings.