Follow HTTP redirects (#328)

- Change HttpClient to follow all redirects except HTTPS to HTTP.
- Run language snippet tests with --no-cache and real PackageServer
  instead of pre-seeded cache.
  This increases HTTP test coverage and enables testing of package redirects.
- Change PackageServer to return 301 for request paths starting with /HTTP301/
  and 307 for request paths starting with /HTTP307/.
- Update some outdated test package checksums that apparently weren't verified.

pkl-core/src/main/java/org/pkl/core/http/JdkHttpClient.java

@@ -22,6 +22,7 @@ import java.lang.invoke.MethodHandles;
 import java.lang.invoke.MethodType;
 import java.net.ConnectException;
 import java.net.URI;
+import java.net.http.HttpClient.Redirect;
 import java.net.http.HttpRequest;
 import java.net.http.HttpResponse;
 import java.net.http.HttpResponse.BodyHandler;
@@ -81,6 +82,7 @@ final class JdkHttpClient implements HttpClient {
         java.net.http.HttpClient.newBuilder()
             .sslContext(createSslContext(certificateFiles, certificateUris))
             .connectTimeout(connectTimeout)
+            .followRedirects(Redirect.NORMAL)
             .build();
   }