Change the way package hashes are calculated

The hash algorithm is as follows for a given package:

• Get the full list of dependencies, remove duplicates, sort alphabetically
• compose a string of the form <name>=<version>-<hash>[/<name2>=<version2>-<hash2>]...
• append in front <name>=<version>[:<git-short-revision>[:<revision>][:<patchfile-hash>]/
• calculate SHA1 of the resulting string and get the 5 first digits

The hash of the packages without dependencies should be the same as before.