Background indexing of package plugins fails #2755

  func testPackagePlugin() async throws {
    let project = try await SwiftPMTestProject(
      files: [
        "Test.swift": "",
        "Plugins/PrintMessage/Plugin.swift": """
        import PackagePlugin

        func 1️⃣callMe() {
        }

        @main
        struct PrintMessagePlugin: CommandPlugin {
          func performCommand(context: PluginContext, arguments: [String]) throws {
            let x: String = 1
          }
        }
        """,
        "Plugins/PrintMessage/Other.swift": """
        func test() {
          callMe()
        }
        """,
      ],
      manifest: """
        import PackageDescription

        let package = Package(
          name: "PrintMessage",
          platforms: [.macOS(.v14)],
          products: [
            .plugin(
              name: "PrintMessage",
              targets: ["PrintMessage"]
            )
          ],
          targets: [
            .target(name: "MyLibrary"),
            .plugin(
              name: "PrintMessage",
              capability: .command(
                intent: .custom(
                  verb: "print-message",
                  description: "Prints message"
                )
              )
            )
          ]
        )
        """,
      enableBackgroundIndexing: true
    )
    let (uri, _) = try project.openDocument("Plugin.swift")
    let diags = try await project.testClient.send(DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri)))
    XCTAssertEqual(
      diags.fullReport?.items.map(\.message),
      ["Cannot convert value of type 'Int' to specified type 'String'"]
    )
    let references = try await project.testClient.send(
      ReferencesRequest(
        textDocument: TextDocumentIdentifier(uri),
        position: project.position(of: "1️⃣", in: "Plugin.swift"),
        context: ReferencesContext(includeDeclaration: false)
      )
    )
    XCTAssertGreaterThan(references.count, 0)
  }