Add use of a Swift Package to test caching

https://bugs.swift.org/browse/SR-11760 might be caused by the loss of fractional timestamps. actions/toolkit#475 should store fractional timestamps as part of the cache, but has not been merged yet.

I expect the cache to not reduce the runtime currently.

.github/workflows/swiftpm-cache-check.yml

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+name: Test SwiftPM Cache
+
+on:
+  push:
+
+jobs:
+  run_xcutils:
+    name: Run xcutils select --print-versions
+    runs-on: macos-latest
+    strategy:
+      matrix:
+        xcode: ["11.5"]
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Select Xcode ${{ matrix.xcode }}
+      run: sudo xcode-select --switch /Applications/Xcode_${{ matrix.xcode }}.app
+
+    - name: Cache SwiftPM
+      uses: actions/cache@v2
+      with:
+        path: .build
+        key: ${{ runner.os }}-xcode_${{ matrix.xcode }}-swiftpm-ci-deps-${{ github.workspace }}-${{ hashFiles('Package.resolved') }}
+        restore-keys: |
+          ${{ runner.os }}-xcode_${{ matrix.xcode }}-swiftpm-ci-deps-${{ github.workspace }}
+
+    - name: Run Tests
+      run: swift run --configuration release --skip-update xcutils select --print-versions

.gitignore

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+/.build

Package.resolved

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+{
+  "object": {
+    "pins": [
+      {
+        "package": "swift-argument-parser",
+        "repositoryURL": "https://github.com/apple/swift-argument-parser",
+        "state": {
+          "branch": null,
+          "revision": "223d62adc52d51669ae2ee19bdb8b7d9fd6fcd9c",
+          "version": "0.0.6"
+        }
+      },
+      {
+        "package": "Version",
+        "repositoryURL": "https://github.com/mxcl/Version.git",
+        "state": {
+          "branch": null,
+          "revision": "200046c93f6d5d78a6d72bfd9c0b27a95e9c0a2b",
+          "version": "1.2.0"
+        }
+      },
+      {
+        "package": "xcutils",
+        "repositoryURL": "https://github.com/JosephDuffy/xcutils.git",
+        "state": {
+          "branch": null,
+          "revision": "0454a5dd89b680a841ada3138cd90f6c606b2d28",
+          "version": "0.1.0"
+        }
+      }
+    ]
+  },
+  "version": 1
+}

Package.swift

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+// swift-tools-version:5.2
+import PackageDescription
+
+let package = Package(
+    name: "CIDependencies",
+    platforms: [
+        .macOS(.v10_10),
+    ],
+    dependencies: [
+        .package(url: "https://github.com/JosephDuffy/xcutils.git", from: "0.1.0"),
+    ],
+    targets: [
+        .target(name: "CIDependencies")
+    ]
+)

Sources/CIDependencies/Empty.swift

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+// An empty file to make this a valid target