Make `RawOrImport` be relative to the config file

Currently, the `RawOrImport` type reads an imported file relative to the directory from which the binary is running, rather than relative to the config file. This should be fixed.

Suppose you have a directory tree that looks like:
```
foo
└── test
├── config.toml
└── setup.toml
```

In the `config.toml`, I should be able to write
```toml
setup = { import = "./setup.toml" }
```
and use `bedrock` from the `foo` directory.

Currently, one must use
```toml
setup = { import = "./test/setup.toml" }
```
in order to use `bedrock` from the foo directory.

Locate the RawOrImport implementation and the config-loading entry point, then trace how an imported path is resolved. Reproduce the foo/test/config.toml and setup.toml scenario from the issue; done means ./setup.toml works when bedrock is run from foo, with the existing behavior covered by a test.

Written by the indexing model from the issue text.