File path should be resolved relative to the source files path

As far as I can tell, currently, the path of the file to be embedded is resolved relative to the compilation root. This doesn't work when the project is in a subfolder of a bigger project, since the root changes. This would be fixed if the file path is always resolved relative to the source file, making it dune root-independent

Start by tracing the PPX's file-embedding path resolution and how it obtains the compilation root versus the source file location. Verify the behavior when the project is in a subfolder of a larger project. Done means embedded file paths resolve relative to the source file and remain independent of the dune root.

Written by the indexing model from the issue text.