Fix broken symlinks (#1988)

Motivation:

Reshuffling the example code in f77ea808 broke some symlinks which
fuzzing relied on.

Modifications:

- Fix broken symlinks
- Bump tools version

Result:

Fuzzing builds

FuzzTesting/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.8
 /*
  * Copyright 2021, gRPC Authors All rights reserved.
  *

FuzzTesting/Sources/EchoImplementation

@@ -1 +1 @@
-../../Sources/Examples/Echo/Implementation
+../../Sources/Examples/v1/Echo/Implementation

FuzzTesting/Sources/EchoModel

@@ -1 +1 @@
-../../Sources/Examples/Echo/Model
+../../Sources/Examples/v1/Echo/Model