Reformat code idents and introduce the SwiftFormat linter (#339)

* Package.swift: add SwiftFormat

Can be invoked with "swift package plugin swiftformat".

* $ swift package plugin swiftformat

* .cirrus.yml: run SwiftFormat

* SwiftFormat: exclude Sources/tart/OCI/Reference/Generated

.cirrus.yml

@@ -22,6 +22,18 @@ task:
     path: "integration-tests/pytest-junit.xml"
     format: junit
 
+task:
+  name: Lint
+  alias: lint
+  macos_instance:
+    image: ghcr.io/cirruslabs/macos-ventura-xcode:latest
+  lint_script:
+    - swift package plugin --allow-writing-to-package-directory swiftformat --cache ignore --lint --report swiftformat.json .
+  always:
+    swiftformat_report_artifacts:
+      path: swiftformat.json
+      format: swiftformat
+
 task:
   name: Build
   alias: build
@@ -37,6 +49,7 @@ task:
   name: Release
   only_if: $CIRRUS_TAG != ''
   depends_on:
+    - lint
     - test
     - build
   macos_instance:

.swiftformat

@@ -0,0 +1,5 @@
+--disable all
+--enable indent
+--indent 2
+--exclude Sources/tart/OCI/Reference/Generated
+--swiftversion 5.7

Package.resolved

@@ -98,6 +98,15 @@
         "revision" : "6190d0cefff3013e77ed567e6b074f324e5c5bf5",
         "version" : "6.3.1"
       }
+    },
+    {
+      "identity" : "swiftformat",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/nicklockwood/SwiftFormat",
+      "state" : {
+        "revision" : "7c7dd06554a5fc3f452f1d16a780a81a3be04bce",
+        "version" : "0.50.4"
+      }
     }
   ],
   "version" : 2

Package.swift

@@ -18,6 +18,7 @@ let package = Package(
     .package(url: "https://github.com/sushichop/Puppy", from: "0.5.1"),
     .package(url: "https://github.com/antlr/antlr4", branch: "dev"),
     .package(url: "https://github.com/apple/swift-atomics.git", .upToNextMajor(from: "1.0.0")),
+    .package(url: "https://github.com/nicklockwood/SwiftFormat", from: "0.50.4"),
   ],
   targets: [
     .executableTarget(name: "tart", dependencies: [