fs/walk.ts: Don't traverse directories matching a skip pattern

In other words, support path prefixes when specifying excludes. This
aligns with the file matchers of the test runner and prettier. It's less
redundant since this cannot be achieved efficiently on the client-end.

fs/glob.ts

@@ -109,9 +109,7 @@ export async function* expandGlob(
   const globOptions: GlobOptions = { extended, globstar, strict };
   yield* walk(root, {
     match: [globToRegExp(absGlob, globOptions)],
-    skip: absExclude.map(
-      (s: string): RegExp => globToRegExp(s, { ...globOptions, flags: "g" })
-    ),
+    skip: absExclude.map((s: string): RegExp => globToRegExp(s, globOptions)),
     includeDirs
   });
 }
@@ -136,9 +134,7 @@ export function* expandGlobSync(
   const globOptions: GlobOptions = { extended, globstar, strict };
   yield* walkSync(root, {
     match: [globToRegExp(absGlob, globOptions)],
-    skip: absExclude.map(
-      (s: string): RegExp => globToRegExp(s, { ...globOptions, flags: "g" })
-    ),
+    skip: absExclude.map((s: string): RegExp => globToRegExp(s, globOptions)),
     includeDirs
   });
 }

fs/walk.ts

@@ -75,6 +75,8 @@ export async function* walk(
   if (options.includeDirs && include(root, options)) {
     const rootInfo = await stat(root);
     yield { filename: root, info: rootInfo };
+  } else if (patternTest(options.skip || [], root)) {
+    return;
   }
   let ls: FileInfo[] = [];
   try {