properly parse the cpanmetadb response

e.g. using 'if' in a dist with starting_version = 5.006 would
cause the build to fail, because _is_dual was returning false due to a failure
to extract the correct field from the response.
avoid blank line in diagnostic output in --verbose mode
issue a stronger warning when a distribution cannot be found in the index, and return undef

This should never actually happen unless there was an error with the query,
because we already looked for the module in Module::CoreList.
cache the results from _is_dual to prevent performing identical network queries
Dist-Zilla-Plugin-OnlyCorePrereqs-0.019

          - fixed bad parsing of response from cpanmetadb, which resulted in
            invalid failures for dual-life upstream-blead modules when
            check_dual_life_versions = 0