[ty] Attach subdiagnostics to unresolved-import errors for relative imports as well as absolute imports (#21554)

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_Using_`from`_with_an…_(6cff507dc64a1bff).snap

@@ -28,6 +28,10 @@ error[unresolved-import]: Cannot resolve imported module `.does_not_exist.foo.ba
 2 |
 3 | stat = add(10, 15)
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
+info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_Using_`from`_with_an…_(9da56616d6332a83).snap

@@ -28,6 +28,10 @@ error[unresolved-import]: Cannot resolve imported module `.does_not_exist`
 2 |
 3 | stat = add(10, 15)
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
+info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_Using_`from`_with_to…_(4b8ba6ee48180cdd).snap

@@ -40,6 +40,10 @@ error[unresolved-import]: Cannot resolve imported module `....foo`
 2 |
 3 | stat = add(10, 15)
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
+info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 
 ```

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -5803,6 +5803,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             ".".repeat(level as usize),
             module.unwrap_or_default()
         ));
+
         if level == 0 {
             if let Some(module_name) = module.and_then(ModuleName::new) {
                 let program = Program::get(self.db());
@@ -5831,39 +5832,39 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     }
                 }
             }
+        }
 
-            // Add search paths information to the diagnostic
-            // Use the same search paths function that is used in actual module resolution
-            let verbose = self.db().verbose();
-            let search_paths = search_paths(self.db(), ModuleResolveMode::StubsAllowed);
+        // Add search paths information to the diagnostic
+        // Use the same search paths function that is used in actual module resolution
+        let verbose = self.db().verbose();
+        let search_paths = search_paths(self.db(), ModuleResolveMode::StubsAllowed);
 
-            diagnostic.info(format_args!(
-                "Searched in the following paths during module resolution:"
-            ));
+        diagnostic.info(format_args!(
+            "Searched in the following paths during module resolution:"
+        ));
 
-            let mut search_paths = search_paths.enumerate();
+        let mut search_paths = search_paths.enumerate();
 
-            while let Some((index, path)) = search_paths.next() {
-                if index > 4 && !verbose {
-                    let more = search_paths.count() + 1;
-                    diagnostic.info(format_args!(
-                        "  ... and {more} more paths. Run with `-v` to see all paths."
-                    ));
-                    break;
-                }
+        while let Some((index, path)) = search_paths.next() {
+            if index > 4 && !verbose {
+                let more = search_paths.count() + 1;
                 diagnostic.info(format_args!(
-                    "  {}. {} ({})",
-                    index + 1,
-                    path,
-                    path.describe_kind()
+                    "  ... and {more} more paths. Run with `-v` to see all paths."
                 ));
+                break;
             }
+            diagnostic.info(format_args!(
+                "  {}. {} ({})",
+                index + 1,
+                path,
+                path.describe_kind()
+            ));
+        }
 
-            diagnostic.info(
-                "make sure your Python environment is properly configured: \
+        diagnostic.info(
+            "make sure your Python environment is properly configured: \
                 https://docs.astral.sh/ty/modules/#python-environment",
-            );
-        }
+        );
     }
 
     fn infer_import_definition(