Relative paths in configurations provided on the command line are relative to configuration directory

For example, if I create a `my_config.yaml` in a directory named `foo` with the following settings:

``` yaml
directory: Music
```

And load it using `beets -c my_config.yaml`, it uses the directory `~/.config/beets/Music` instead of `foo/Music` for the directory.

Start by tracing how the `beets -c my_config.yaml` command loads configuration and resolves the `directory` setting. Reproduce the example with `foo/my_config.yaml` and verify that the effective path is `foo/Music` rather than `~/.config/beets/Music`.

Written by the indexing model from the issue text.