dev-cmd/extract: allow maintainers to extract to core.

I was wanting to do this today to add `ruby@2.6` and it's a social
rather than technical requirement (i.e. we don't want users to mess
with their homebrew/core unless they know what they are doing) so allow
overriding with `HOMEBREW_DEVELOPER` set.

Library/Homebrew/dev-cmd/extract.rb

@@ -111,8 +111,10 @@ module Homebrew
     end
 
     destination_tap = Tap.fetch(args.remaining.second)
+    unless ARGV.homebrew_developer?
       odie "Cannot extract formula to homebrew/core!" if destination_tap.core_tap?
       odie "Cannot extract formula to the same tap!" if destination_tap == source_tap
+    end
     destination_tap.install unless destination_tap.installed?
 
     repo = source_tap.path