Improve reporting for doc links problems

crates/compiler/module/src/ident.rs

@@ -1,5 +1,8 @@
 pub use roc_ident::IdentStr;
-use std::fmt::{self, Debug};
+use std::{
+    fmt::{self, Debug},
+    path::{Path, PathBuf},
+};
 
 use crate::symbol::PQModuleName;
 
@@ -45,6 +48,19 @@ impl<'a> QualifiedModuleName<'a> {
 #[derive(Clone, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct ModuleName(IdentStr);
 
+impl ModuleName {
+    /// Given the root module's path, infer this module's path based on its name.
+    pub fn filename(&self, root_filename: impl AsRef<Path>) -> PathBuf {
+        let mut answer = root_filename.as_ref().with_file_name("");
+
+        for part in self.split(".") {
+            answer = answer.join(part);
+        }
+
+        answer.with_extension("roc")
+    }
+}
+
 impl std::ops::Deref for ModuleName {
     type Target = str;