Workaround SwiftPM API breakage checker limitation (#1211)

### Motivation In order to address a hanging integration test on newer Swift versions, #1208 added a target dependency on the executable used in the multi-node test plugin and stopped shelling out to `swift run`. This addressed the hang, but it has surfaced another issue: that SwiftPM's API breakage checker has issues with some packages that contain plugins, and now that pipeline is reproducibly resulting in false positives. ### Modifications This patch removes that dependency in the package manifest and reverts to running it from within the plugin. There was already code in the plugin to build the package and locate the required binary in the built artifacts, which I hadn't noticed in the previous patch, so the use of `swift run` seemed unnecessary. This patch just executes that found executable without `swift run`, which (hopefully) resolves both issues. ### Result: - Integration test should run to completion on all Swift versions (not hang on 6+) - API breakage pipeline should pass.

Author: simonjbeaumont

Package.swift

@@ -93,7 +93,10 @@ var targets: [PackageDescription.Target] = [
  // permissions: needs full network access
  ),
  dependencies: [
- "MultiNodeTestKitRunner"
+ /// NOTE: Usually a Swift package plugin would declare any internal executable targets it depends on here
+ /// and make use of them from the context passed in by SwiftPM. However, this causes issues when
+ /// running the API breakage checker on this package, for unrelated reasons. So this command plugin is
+ /// building the package itself as part of its run.
  ]
  ),
  .target(

Plugins/MultiNodeTestPlugin/plugin.swift

@@ -39,11 +39,10 @@ final class MultiNodeTestPlugin: CommandPlugin {
  }
  }
 
- // Find the executable dependency frome the plugin context.
- let tool = try context.tool(named: "MultiNodeTestKitRunner")
+ let toolName = "MultiNodeTestKitRunner"
 
  // Terminate all previous runners
- Process.killall(name: tool.name)
+ Process.killall(name: toolName)
 
  switch self.buildConfiguration {
  case .debug:
@@ -66,22 +65,19 @@ final class MultiNodeTestPlugin: CommandPlugin {
 
  let multiNodeRunner = buildResult.builtArtifacts
  .filter { $0.kind == .executable }
- .first { $0.path.lastComponent.starts(with: "MultiNodeTestKitRunner") }
+ .first { $0.path.lastComponent.starts(with: toolName) }
  guard let multiNodeRunner = multiNodeRunner else {
  throw MultiNodeTestPluginError(message: "Failed")
  }
 
  log("Detected multi-node test runner: \(multiNodeRunner.path.lastComponent)")
 
  let process = Process()
- process.binaryPath = tool.path.string
- process.arguments = []
- for arg in arguments {
- process.arguments?.append(arg)
- }
+ process.binaryPath = multiNodeRunner.path.string
+ process.arguments = arguments
 
  do {
- log("> \(tool.name) \(process.arguments?.joined(separator: " ") ?? "")")
+ log("> \(toolName) \(process.arguments?.joined(separator: " ") ?? "")")
  try process.runProcess()
  process.waitUntilExit()
  } catch {