multiple default_sources should have a configurable priority for automatic preferred_for behavior

The default behavior should not change, but users really want to be able to say "use all my :chef_repo cookbooks first, then use :supermarket second, and i don't want to tell you that my version of build-essentials should always take priority"

Right now that looks something like:

default_source :chef_repo, “./cookbooks” do |s|
  s.preferred_for *(Dir.entries(‘./cookbooks’) - [‘.’, ‘..’] & self.run_list.map {|c| c.split(‘::’).first})
end

The new DSL code should behave the same way so that if the :chef_repo takes priority over :supermaket then the presence of a build-essential-0.0.1 cookbook in the :chef_repo would take precedence over any version of build-essential in supermarket and there would be no crazy merging of actual versions -- presence of any version in a higher priority repo would knock out all versions in a lower priority repo.

Start by locating the DSL implementation for default_source and preferred_for, then trace how cookbook versions are selected across multiple sources. Preserve the current behavior by default, while making source priority configurable so that any cookbook in a higher-priority source excludes all versions from lower-priority sources. Add or update coverage for the chef_repo-before-supermarket example described in the issue.