Improve fp module annotations to automatically derive module path

[sagacity]

Right now fp annotations that end with *_module (e.g. fp(rust_plugin_module = "my_crate::types")) add quite some verbosity since the module path of the original types needs to be explicity specified. We should consider if we can generate this information automatically.

There is currently a nightly-only feature that allows us to retrieve the source_file of a Span, but this is not available in stable Rust yet. Once this is available, we can clean up the annotation.