Don't import ECS fields of type group

[jsoriano]

elastic-package build doesn't import anymore fields of type group, or groups without subfields.

Before this change, it imported fields of type group and all its subfields, what was unexpected (see elastic/package-spec#331).

A check is also added to don't allow to store values in names of fields of type group.

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-05-12T11:47:41.810+0000

• Duration: 33 min 28 sec

Test stats 🧪

Test	Results
Failed	0
Passed	703
Skipped	0
Total	703

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (30/30)	💚
Files	64.22% (70/109)	👍
Classes	58.278% (88/151)	👍
Methods	47.555% (282/593)	👍 0.089
Lines	32.282% (2601/8057)	👍 0.287
Conditionals	100.0% (0/0)	💚