Refactor transform package format

[jonathan-buttner]

The transform format currently requires a directory and a file: https://github.com/elastic/endpoint-package/tree/master/package/endpoint/elasticsearch/transform/metadata_current

For example the link above requires the directory metadata_current and the actual transform file default.json. This could probably be refactored to only use a file. So the package path would look like:

elasticsearch/transform/metadata_current_blah.json.

[ruflin]

++ on cleaning this up. I think it is an onversight that transform contained an additional sub directory. It should be aligned with all the other assets. Would be nice if we find a way to clean this up without having to keep too much legacy code / spec definition.

[joshdover]

Transforms now include their associated field list, this is no longer relevant as of #307