Generate zplugin.go correctly with third-party plugins (#6692)

Previously, the generation of zplugin.go would not separate
third-party external plugins from CoreDNS plugins in the go
import block. This leads to the TestImportOrdering unit test
failing, as it requires that third-party imports paths to be
in a separate import block.

While this issue does not affect the main CoreDNS repo, it can cause
unit test failures in forks of CoreDNS that include external third-party
plugins.

Signed-off-by: Grant Spence <gspence@redhat.com>

directives_generate.go

@@ -58,10 +58,22 @@ func genImports(file, pack string, mi map[string]string) {
 		outs += "\n"
 	}
 
+	coreDnsImports := ""
+	thirdPartyImports := ""
+
 	outs += "// Include all plugins.\n"
 	for _, v := range mi {
-		outs += `_ "` + v + `"` + "\n"
+		if strings.HasPrefix(v, githubOrg) {
+			coreDnsImports += `_ "` + v + `"` + "\n"
+		} else {
+			thirdPartyImports += `_ "` + v + `"` + "\n"
+		}
 	}
+	outs += coreDnsImports
+	if thirdPartyImports != "" {
+		outs += "\n" + thirdPartyImports
+	}
+
 	outs += ")\n"
 
 	if err := formatAndWrite(file, outs); err != nil {
@@ -107,7 +119,8 @@ func formatAndWrite(file string, data string) error {
 }
 
 const (
-	pluginPath   = "github.com/coredns/coredns/plugin/"
+	githubOrg    = "github.com/coredns"
+	pluginPath   = githubOrg + "/coredns/plugin/"
 	pluginFile   = "plugin.cfg"
 	pluginFSPath = "plugin/" // Where the plugins are located on the file system
 	header       = "// generated by directives_generate.go; DO NOT EDIT\n\n"