fix various snapmatch issues

• when version match score is 0, overall score is still calculated as 50, so snapmatch tries to download info about a non-existent "" version (empty string) from Debian Snapshot API
• libpam is not found, but it corresponds to "pam" package in Debian, need to set an alias
• there is an error when trying to display some log messages, because a dict key is treated as an object property

fix #90 (closed)