unique: implement custom version of unique package

This version probably isn't as fast as the upstream version, but it is
good enough for now. It also doesn't free unreferenced handles like the
upstream version.

loader/goroot.go

@@ -251,6 +251,7 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"runtime/":              false,
 		"sync/":                 true,
 		"testing/":              true,
+		"unique/":               false,
 	}
 
 	if goMinor >= 19 {