Reorganize templates in subdirectories about templates

[djbe]

While I've implemented this in this repo, while implementing this on the SwiftGen repo's side (CLI), it becomes a breaking change:
The CLI will look in both the internal templates directory, but also in the application support templatesdirectory. If we change the code for how it searches for templates, we should make it consistent for both directories (internal and app support).

This is the patch:

-  var path = appSupportTemplatesPath + "\(prefix)-\(templateShortName).stencil"
+  var path = appSupportTemplatesPath + prefix + "\(templateShortName).stencil"
   if !path.isFile {
-    path = bundledTemplatesPath + "\(prefix)-\(templateShortName).stencil"
+    path = bundledTemplatesPath + prefix + "\(templateShortName).stencil"

[AliSoftware]

Note that if we do that in SwiftGen 5.0, we must not forget to update the help message for swiftgen templates list accordingly as well.