Add test and update reference.

tests/ui/single_component_path_imports.fixed

@@ -0,0 +1,12 @@
+// run-rustfix
+// compile-flags: --edition 2018
+#![warn(clippy::single_component_path_imports)]
+#![allow(unused_imports)]
+
+
+use serde as edres;
+pub use serde;
+
+fn main() {
+    regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+}

tests/ui/single_component_path_imports.rs

@@ -0,0 +1,12 @@
+// run-rustfix
+// compile-flags: --edition 2018
+#![warn(clippy::single_component_path_imports)]
+#![allow(unused_imports)]
+
+use regex;
+use serde as edres;
+pub use serde;
+
+fn main() {
+    regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+}

tests/ui/single_component_path_imports.stderr

@@ -0,0 +1,10 @@
+error: this import is redundant
+  --> $DIR/single_component_path_imports.rs:6:1
+   |
+LL | use regex;
+   | ^^^^^^^^^^ help: remove it entirely
+   |
+   = note: `-D clippy::single-component-path-imports` implied by `-D warnings`
+
+error: aborting due to previous error
+