compiler: Rework path handling

Add some code to do platform-independent path processing.

This is necessary aas WASM does e.g. not have any absolute paths and
such and the compiler tended to produce wrong results in that case.

Side-effect: We no longer need to depend on `dunce`

internal/compiler/lib.rs

@@ -30,6 +30,7 @@ pub mod lookup;
 pub mod namedreference;
 pub mod object_tree;
 pub mod parser;
+pub mod pathutils;
 pub mod typeloader;
 pub mod typeregister;