More robust optional package handling
- parsing of EXOMETER_PACKAGES handles extraneous quotes
- defines, e.g. `dep_afunix` added to indicate which optional apps
  are included
- exometer_report_amqp.erl checks for dep_amqp_client and uses
  copy-pasted record defs otherwise, just to make it compile.
- Some dialyzer warnings fixes in exometer_report_amqp.erl