Handle indented import groups

Author: MattX

src/formatting/reorder.rs

@@ -256,7 +256,8 @@ fn rewrite_reorderable_or_regroupable_items(
                 })
                 .collect::<Option<Vec<_>>>()?;
 
-            Some(item_vec.join("\n\n"))
+            let join_string = format!("\n\n{}", shape.indent.to_string(context.config));
+            Some(item_vec.join(&join_string))
         }
         _ => {
             let list_items = itemize_list(

tests/source/configs/group_imports/StdExternalCrate-nested.rs

@@ -0,0 +1,6 @@
+// rustfmt-group_imports: StdExternalCrate
+mod test {
+    use crate::foo::bar;
+    use std::path;
+    use crate::foo::bar2;
+}

tests/target/configs/group_imports/StdExternalCrate-nested.rs

@@ -0,0 +1,7 @@
+// rustfmt-group_imports: StdExternalCrate
+mod test {
+    use std::path;
+
+    use crate::foo::bar;
+    use crate::foo::bar2;
+}