`dub template` command does not work with relative source paths

Passing a relative path as a source to `dub template` breaks, since the path is not made absolute before passing it to the Jinja template loader, which is only configured with the root (`/`) in it's search path. The path should be made absolute before use, or the current directory should be added to the search path.

Locate the entry point for the `dub template` command and trace how its source path is passed to the Jinja template loader. Verify the fix by running `dub template` with a relative source path and confirm that template generation succeeds.

Written by the indexing model from the issue text.