add schema for Perl 6 tables

... so we are able to run tests (t/mldistwatch.t).
do not send misleading mails for P6 dists
fix detection of META6.json and sql inserts

We get a string containing all lines back from tar, the previous code
expeted to loop over the output line by line.
The calls to $dbh->do expect a hashref as second argument, we can
pass undef here.
add mldistwatch test for Perl 6 indexing