Fix fileset field prefixing (#20170)

When the fields.yml file is constructed it is done by appending files together and adding some indenting.
In the case of Filebeat, a fileset's fields.yml is appended with an indent of 8 spaces to the module's fields.yml.
This generally allows for all of the filesets fields to become children of the module.

The problem we had was that the new filesets added in #19713 expected that their fields would be root fields
(not children to the module namespace). In cases where the module already existed and had declared
a module namespace field in its fields.yml this resulted in unexpectedly namespaced fieldset fields
(e.g. microsoft.rsa.* instead of rsa.*).

The size of the x-pack/filebeat index-pattern is still large (915885 bytes), but not so large that it goes beyond the Kibana
request payload limit.

Fixes #19965

x-pack/filebeat/module/cisco/_meta/fields.yml

@@ -3,8 +3,4 @@
   description: >
     Module for handling Cisco network device logs.
   fields:
-    - name: cisco
-      type: group
-      description: >
-        Fields from Cisco logs.
-      fields:
+

x-pack/filebeat/module/cisco/asa/_meta/fields.yml

@@ -1,4 +1,4 @@
-- name: asa
+- name: cisco.asa
   type: group
   description: >
     Fields for Cisco ASA Firewall.

x-pack/filebeat/module/cisco/ftd/_meta/fields.yml

@@ -1,4 +1,4 @@
-- name: ftd
+- name: cisco.ftd
   type: group
   description: >
     Fields for Cisco Firepower Threat Defense Firewall.

x-pack/filebeat/module/cisco/ios/_meta/fields.yml

@@ -1,4 +1,4 @@
-- name: ios
+- name: cisco.ios
   type: group
   description: >
     Fields for Cisco IOS logs.

x-pack/filebeat/module/fortinet/_meta/fields.yml

@@ -3,12 +3,3 @@
   description: >
     fortinet Module
   fields:
-    - name: fortinet
-      type: group
-      description: >
-        Fields from fortinet FortiOS
-      fields:
-        - name: file.hash.crc32
-          type: keyword
-          description: >
-            CRC32 Hash of file