.golangci.yml- Disallow OSS -> x-pack dependencies

According to LICENSE.txt, the Beats project uses both Apache 2.0 (OSS)
and Elastic licenses. The OSS code may not depend on the Elastic
License code otherwise the OSS-only binaries become tainted.

This makes golangci-lint emit warnings for OSS code that depends on
x-pack code.

Closes elastic#38719

.golangci.yml

@@ -66,9 +66,19 @@ linters:
     - wastedassign # wastedassign finds wasted assignment statements.
     - gomodguard # check for blocked dependencies
     - prealloc # Finds slice declarations that could potentially be pre-allocated
+    - depguard
 
 # all available settings of specific linters
 linters-settings:
+  depguard:
+    rules:
+      apache-licensed-code:
+        list-mode: lax
+        files:
+          - '!**/x-pack/**/*.go'
+        deny:
+          - pkg: github.com/elastic/beats/v7/x-pack
+            desc: Apache 2.0 licensed code cannot depend on Elastic licensed code (x-pack/).
   errcheck:
     # report about not checking of errors in type assertions: `a := b.(MyStruct)`;
     check-type-assertions: false