[Change Proposal] Skip validation of required fields on packages importing full ECS mappings

[jsoriano]

Packages using import_mappings: true for ECS, or the ecs@mappings template included in the stack will have definitions for the required fields.

Consider skipping validation of required fields on these packages, taking into account that it may be safer to force developers to make a conscious decision on these fields, most of them are constant keywords that could benefit from a pre-defined value.