chore: refactor linter settings and file structure

- Remove `deadcode` linter and `structcheck` from `.golangci.yml`
- Add settings for `depguard` linter in `.golangci.yml`
- Rename `example/main.go` to `_example/main.go`

Signed-off-by: appleboy <[email protected]>

Author: appleboy

.golangci.yml

@@ -4,7 +4,6 @@ linters:
   fast: false
   enable:
     - bodyclose
-    - deadcode
     - depguard
     - dogsled
     - dupl
@@ -29,15 +28,31 @@ linters:
     - nolintlint
     - rowserrcheck
     - staticcheck
-    - structcheck
     - stylecheck
     - typecheck
     - unconvert
     - unparam
     - unused
-    - varcheck
     - whitespace
     - gofumpt
 
+linters-settings:
+  depguard:
+    rules:
+      Main:
+        files:
+          - $all
+          - "!$test"
+        allow:
+          - $gostd
+          - github.com/gin-gonic/gin
+      Test:
+        files:
+          - $test
+        allow:
+          - $gostd
+          - github.com/gin-gonic/gin
+          - github.com/stretchr/testify
+
 run:
   timeout: 3m