Benchmarks: add PackageGraphBenchmarks target (#7283)

This adds a dependency on the
[`package-benchmark`](https://github.com/ordo-one/package-benchmark)
library, isolated to the new `Benchmarks` folder. Thus it doesn't touch
CMake or self-hosted builds, and one can only pull the library if they
run `swift package benchmark` in the `Benchmarks` directory.

A single benchmark is added that loads SwiftPM's `Package.swift` and
builds a package graph for it.

Author: MaxDesiatov

Benchmarks/Benchmarks/PackageGraphBenchmarks/PackageGraphBenchmarks.swift

@@ -0,0 +1,22 @@
+import Basics
+import Benchmark
+import PackageModel
+import Workspace
+
+let benchmarks = {
+    Benchmark(
+        "Package graph loading",
+        configuration: .init(
+            metrics: BenchmarkMetric.all,
+            maxDuration: .seconds(10)
+        )
+    ) { benchmark in
+        let path = try AbsolutePath(validating: #file).parentDirectory.parentDirectory.parentDirectory
+        let workspace = try Workspace(fileSystem: localFileSystem, location: .init(forRootPackage: path, fileSystem: localFileSystem))
+        let system = ObservabilitySystem { _, _ in }
+
+        for _ in benchmark.scaledIterations {
+            try workspace.loadPackageGraph(rootPath: path, observabilityScope: system.topScope)
+        }
+    }
+}

Benchmarks/Package.swift

@@ -0,0 +1,38 @@
+// swift-tools-version: 5.7
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import PackageDescription
+
+let package = Package(
+    name: "SwiftPMBenchmarks",
+    platforms: [
+        .macOS("14"),
+    ],
+    dependencies: [
+        .package(path: "../"),
+        .package(url: "https://github.com/ordo-one/package-benchmark.git", from: "1.13.0"),
+    ],
+    targets: [
+        .executableTarget(
+            name: "PackageGraphBenchmarks",
+            dependencies: [
+                .product(name: "Benchmark", package: "package-benchmark"),
+                .product(name: "SwiftPMDataModel", package: "SwiftPM"),
+            ],
+            path: "Benchmarks/PackageGraphBenchmarks",
+            plugins: [
+                .plugin(name: "BenchmarkPlugin", package: "package-benchmark"),
+            ]
+        ),
+    ]
+)