pre-resolve package path for lazy-load configuration
pre-resolve package path for lazy-load configuration

`test_ui.ConfigTest.test_command_line_option_relative_to_working_dir` was
failing because it changes working directories before the relative lazy-loaded
default configuration path is resolved (`../beets/__init__.py`), later causing a
`beets.util.confit.NotFoundError`. The Configuration object has been modified
so that lazy-load configurations resolve the path in advance.

sampsyo/confit,sampsyo/confuse