Enable check to avoid duplicated fields

[jsoriano]

This is a follow up of #309.

We have found some issues we have to investigate. It seems that elastic-package is importing more fields than expected when using external: ecs, what complicates checking for fields duplication.
Also, fields validator in elastic-package uses only the fields defined in the source files, so it is sometimes not possible to pass both the validation of defined fields, and the validation to avoid duplicated fields. Original idea of the fields importer was to import only fields defined explicitly and not subfields of groups, so it is ok if fields validator only uses the source files.

So current plan is going to be:

• Disable check for fields duplication and release package-spec 1.8.1 without this. Disable check to avoid duplicated fields #332
• Revert Generate documentation for imported external nested fields elastic-package#814 as it shouldn't be needed. Revert "Generate documentation for imported external nested fields" elastic-package#817
• Make elastic-package to import only fields explicitly configured, and not subfields. Don't import ECS fields of type group elastic-package#818
• Remove duplicated fields from integration packages using a similar approach to the one to remove invalid ECS values (Remove duplicated fields definitions integrations#3392). Update: Waiting for Introduce new package spec version  #345, to do it package per package.
• Once everything is clear, enable the check again (Re-enable validation of duplicated fields #338).

[jsoriano]

Blocking this issue till we have #345