Add top-level completion for path mapping (#21686) (#21689)

Author: Andy

src/services/pathCompletions.ts

@@ -187,7 +187,7 @@ namespace ts.Completions.PathCompletions {
 
         const pathPrefix = path.slice(0, path.length - 1);
         if (!startsWith(fragment, pathPrefix)) {
-            return emptyArray;
+            return [pathPrefix];
         }
 
         const remainingFragment = fragment.slice(pathPrefix.length);

tests/cases/fourslash/completionsPaths_pathMapping_topLevel.ts

@@ -0,0 +1,17 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /x/src/a.ts
+////import {} from "[|/**/|]";
+
+// @Filename: /x/tsconfig.json
+////{
+////    "compilerOptions": {
+////        "baseUrl": ".",
+////        "paths": {
+////            "foo/*": ["src/*"]
+////        }
+////    }
+////}
+
+const [replacementSpan] = test.ranges();
+verify.completionsAt("", ["src", "foo/"].map(name => ({ name, replacementSpan })));