fix snap match

• remove multiprocessing
• fix logging, output and cache policies
  • errors must be logged only when something's gone wrong, and not simply when a match cannot be found or searched for whatever reason (in such cases, a warning must be logged instead)
  • snapmatcher should always return a json output, even when a match cannot be searched or found
  • pool cache must be used if user settings mandate it, but csv stats should be generated in any case
• use aliases in package name matching
• download debian source files
• return debsrc_origin, debsrc_debian and debian_format as the match command (such data are needed for subsequent commands of a4f workflow)

it closes #19 (closed) and #23 (closed)