Replace magic test port 12110 with 0 (#345)

Also introduce constants for some test package SHAs
to make them easier to update.

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

@@ -111,8 +111,8 @@ public interface HttpClient extends AutoCloseable {
     /**
      * Sets a test server's listening port.
      *
-     * <p>If set, requests that specify port 12110 will be modified to use the given port. This is
-     * an internal test option.
+     * <p>If set, requests that specify port 0 will be modified to use the given port. This is an
+     * internal test option.
      */
     Builder setTestPort(int port);
 

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

@@ -105,9 +105,7 @@ final class RequestRewritingClient implements HttpClient {
   }
 
   private URI rewriteUri(URI uri) {
-    // Would be nice to use port 0 instead of 12110,
-    // but this is best done in a separate commit.
-    if (testPort != -1 && uri.getPort() == 12110) {
+    if (testPort != -1 && uri.getPort() == 0) {
       return HttpUtils.setPort(uri, testPort);
     }
     return uri;