Relative paths in the config file are not handled as expected with a symlink

[JakeHillion]

It's common for us to build a "config pack" involving a config file and various auxiliary files like type configurations and headers. Part of this working is allowing relative paths from the config file to these auxiliary files.

This does not work as expected when symlinking. For example:

$ ln -s $CONFIG_PACK/config.oid.toml build/testing.oid.toml

Fails to locate the headers and type configurations in the file.

We can use the C++ APIs to resolve the symlink and create the relative paths accordingly. This should be a simple change.