Validate built packages too

[jsoriano]

Built packages can have content that was not in their source files, such
as imported ECS fields. Validate final packages to ensure that content
included during build is also valid.

For example while testing ValidateFromZip I found that some packages
contained ECS fields of type match_text_only, that was not supported
in the spec (fixed in elastic/package-spec#284).

@mtojek not sure if this is the best place to do this validation, but I think
we should have something like this safeguard to ensure that built packages
still comply with the spec.

[elasticmachine]

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• Start Time: 2022-03-14T12:28:15.047+0000

• Duration: 16 min 3 sec

Test stats 🧪

Test	Results
Failed	0
Passed	536
Skipped	1
Total	537

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[mtojek]

LGTM!

nit: if you haven't done it, you can double-check if command usage messages and "README" are up-to-date in terms of building&linting.

[jsoriano]

nit: if you haven't done it, you can double-check if command usage messages and "README" are up-to-date in terms of building&linting.

General behaviour doesn't change, so I guess we are fine there.