Bind PackageServer to ephemeral port to avoid port conflicts (#227)

This is a comprehensive solution to the "flaky PackageServer tests"
problem. It rules out port conflicts and imposes no limits on test
parallelism. The same solution can be used for other test servers
in the future.

Major changes:
- Turn `PackageServer` from a singleton into a class that is
  instantiated per test class or test method.
- Start the server the first time its `port` property is read.
  Bind the server to an ephemeral port instead of port 12110.
- For every test that uses `PackageServer`, pass the server port to
  `--test-port`, `HttpClient.Builder.setTestPort`, the `CliBaseOptions`
  or `ExecutorOptions` constructor, or the Gradle plugin's `testPort` property.
  Wire all of these to `RequestRewritingClient`'s `testPort` constructor parameter.
- Enhance `RequestRewritingClient` to replace port 12110 with `testPort`
  in request URIs unless `testPort` is -1 (its default).
- Introduce `ExecutorOptions.Builder`.
  This makes executor options more comfortable to create
  and allows to hide options such as `testPort`.
- Deprecate the `ExecutorOptions` constructor to steer users towards the builder.
- Get rid of `ExecutorOptions2`, which is no longer needed.
- Clean up `EmbeddedExecutorTest` with the help of the builder.

pkl-commons-cli/src/main/kotlin/org/pkl/commons/cli/CliBaseOptions.kt

@@ -113,6 +113,12 @@ data class CliBaseOptions(
   /** Tells whether to run the CLI in test mode. This is an internal option. */
   val testMode: Boolean = false,
 
+  /**
+   * Unless -1, rewrites HTTP requests that specify port 12110 to the given port. This is an
+   * internal test option.
+   */
+  val testPort: Int = -1,
+
   /**
    * The CA certificates to trust.
    *
@@ -179,6 +185,7 @@ data class CliBaseOptions(
    */
   val httpClient: HttpClient by lazy {
     with(HttpClient.builder()) {
+      setTestPort(testPort)
       if (normalizedCaCertificates.isEmpty()) {
         addDefaultCliCertificates()
       } else {

pkl-commons-cli/src/main/kotlin/org/pkl/commons/cli/commands/BaseOptions.kt

@@ -17,6 +17,7 @@ package org.pkl.commons.cli.commands
 
 import com.github.ajalt.clikt.parameters.groups.OptionGroup
 import com.github.ajalt.clikt.parameters.options.*
+import com.github.ajalt.clikt.parameters.types.int
 import com.github.ajalt.clikt.parameters.types.long
 import com.github.ajalt.clikt.parameters.types.path
 import java.io.File
@@ -135,6 +136,13 @@ class BaseOptions : OptionGroup() {
       .path()
       .multiple()
 
+  // hidden option used by native tests
+  private val testPort: Int by
+    option(names = arrayOf("--test-port"), help = "Internal test option", hidden = true)
+      .single()
+      .int()
+      .default(-1)
+
   fun baseOptions(
     modules: List<URI>,
     projectOptions: ProjectOptions? = null,
@@ -155,6 +163,7 @@ class BaseOptions : OptionGroup() {
       moduleCacheDir = cacheDir ?: defaults.normalizedModuleCacheDir,
       noCache = noCache,
       testMode = testMode,
+      testPort = testPort,
       omitProjectSettings = projectOptions?.omitProjectSettings ?: false,
       noProject = projectOptions?.noProject ?: false,
       caCertificates = caCertificates