Parse rules and actions yaml headers like we do with content headers

[This is how it's currently done](https://github.com/foundpatterns/lighttouch-libs/blob/7eb07b2262d7cf37db1be123989e6f282b762969/package_loader.lua#L82-L91) (and for [actions](https://github.com/foundpatterns/lighttouch-libs/blob/master/package_loader.lua#L192-L202)), the problem is that it breaks if the header is not exactly how the package manager expects it, even if it's valid yaml.

[This is how it should be done](https://github.com/foundpatterns/lighttouch-keys/blob/1a3195788efeb24bf522822a4ace81c83ad17f40/helpers.lua#L19) (gotten [here](https://github.com/foundpatterns/lighttouch-keys/blob/1a3195788efeb24bf522822a4ace81c83ad17f40/helpers.lua#L7)), it works for any yaml header as long as it's separated with `...` from the body.