Neither the source line nor filename matter for node UUIDs.

Source lines and filenames are relevant to the _input_, not the
_survey_.  You can have two identical surveys with different formatting
and differing origin:

    # from one.rb
    survey "one" do
      section "foo" do
      end
    end

    # from curl https://foo.example.edu/surveys/one | bin/lunokhod-driver
    survey "one" do

      section "foo" do
      end
    end

Closes #3.