Add "encoding/json" to denylist (#18)

- "encoding/json" should be avoided as we got a json module for that which get the correct json library when specified.
- Allow to exemptions to the denylist (e.g. the json module need to import this package).

Reviewed-on: https://gitea.com/gitea/gitea-vet/pulls/18
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-committed-by: Gusted <williamzijl7@hotmail.com>

checks/denylisted-imports.go

@@ -11,7 +11,7 @@ import (
 )
 
 var (
-	deniedImports   = []string{"io/ioutil"}
+	deniedImports   = []string{"io/ioutil", "encoding/json"}
 	DenylistImports = &analysis.Analyzer{
 		Name: "denylist_imports",
 		Doc:  "check for denied imports",
@@ -27,7 +27,18 @@ func runDenylistImports(pass *analysis.Pass) (interface{}, error) {
 			val = strings.TrimSuffix(val, `"`)
 			for _, deniedImport := range deniedImports {
 				if deniedImport == val {
-					pass.Reportf(im.Path.Pos(), `"`+deniedImport+"\" is not allowed to be imported")
+					// Allow a exemption when there is a comment 'Allow "package_name" import'
+					allowed := false
+					for _, comment := range file.Comments {
+						if strings.Contains(comment.Text(), "Allow \""+val+"\" import") {
+							allowed = true
+							break
+						}
+					}
+
+					if !allowed {
+						pass.Reportf(im.Path.Pos(), `"`+deniedImport+"\" is not allowed to be imported")
+					}
 				}
 			}
 		}