Fix plugin template resolution (#2733)

- According to the documentation comment for [templates.Options], if the
  `Template` and `TemplateFS` fields are empty, it `Render` should find
  the `.gotpl` files from the calling plugin. However, it looks like
  this is broken since #2262 moved the resolution code to a separate
  helper function. This results in broken behavior in consumers such as
  [infiotinc/gqlgenc](https://github.com/infiotinc/gqlgenc) when they
  use the latest version of `gqlgen` as instead of finding the template
  from the plugin, the test template from this package is used which
  outputs only: `this is my test package`.
- The cause for this is that `runtime.Caller` was still only skipping
  one stack level which means that it was finding the `Render` function
  instead of its caller.

[templates.Options]: https://pkg.go.dev/github.com/99designs/gqlgen@v0.17.35/codegen/templates#Options

codegen/templates/templates.go

@@ -172,7 +172,7 @@ func parseTemplates(cfg Options, t *template.Template) (*template.Template, erro
 		fileSystem = cfg.TemplateFS
 	} else {
 		// load path relative to calling source file
-		_, callerFile, _, _ := runtime.Caller(1)
+		_, callerFile, _, _ := runtime.Caller(2)
 		rootDir := filepath.Dir(callerFile)
 		fileSystem = os.DirFS(rootDir)
 	}