AlienPackage.expand() method returns error when checking aliensrc pkgs with tagged source files

Before adding the whole thing about tagged source files in aliensrc packages, we added a check to ensure that the source files listed in aliensrc.json match the actual files found in the archive. However, after adding the possibility to tag source files (when they are used only for only one or only some distros or target machines), and taking into account that the same file name now may refer to different files with different checksums depending on the machine (eg. xorg.conf, which is different depending on the target machine), and that we can have the same file with multiple tag paths that may consequently appear more than once in the tarball, we cannot do the check in this way any more, because it returns errors when there are tagged files; instead, we should just compare the set of the file checksums listed in aliensrc.json with the set of actual checksums of the files contained in the archive