Cherry-pick #14498 to 7.x: Replace generate_imports python script with mage imports

[ChrsMark]

Cherry-pick of PR #14498 to 7.x branch. Original message:

This PR tries to refactor and unify the way we create include/list.go files. This came after we moved light modules to OSS and while trying to create a light module under OSS. In that case this module will be included in list.go file despite the fact that it has no .go files. This was not a problem when light modules where under x-pack because there the list.go was created by mage and the script was taking into consideration the existence of .go files in a module so as to decide if it should be included in list.go.

So far we had:

• x-pack way: https://github.com/elastic/beats/blob/master/x-pack/metricbeat/magefile.go#L128
• oss way: https://github.com/elastic/beats/blob/master/metricbeat/Makefile#L46

This PR refactors the OSS part so as to make use of the mage respective target likewise x-pack.

cc: @jsoriano , @exekias