rustdoc: Add missing src links for generic impls on trait pages

`implementor2item` would return `None` for generic impls so instead this clones the entire `clean::Item` into the `implementors` map which simplifies some code.

src/test/rustdoc/issue-43893.rs

@@ -22,3 +22,8 @@ impl SomeTrait for usize {}
 impl SomeTrait for SomeStruct {
     // deliberately multi-line impl
 }
+
+pub trait AnotherTrait {}
+
+// @has foo/trait.AnotherTrait.html '//a/@href' '../src/foo/issue-43893.rs.html#29'
+impl<T> AnotherTrait for T {}