Use full namespace for builder field
zef has special-cased 'MakeFromJSON' for the time being. Some time in the near future you should be able to merge this with little fallout.
@niner how shall we handle this in install-dist.pl? Do we do my $builder-class = first { *.so }, (try require ::("$name")), (try require ::("Distribution::Builder::$name") with a deprecation warning, or should we just cut straight to ::("$name")?
install-dist.p6 will handle a full namespace or the original implied prefix version as of https://github.com/rakudo/rakudo/commit/0249afc26fbac525969ef19e52da33cd794ba275 and the spec updated as of https://github.com/perl6/specs/commit/26266b1bc69cd15b58ab03338af7f3938052b666 to reflect that using the full namespace is the new correct.