Fix backwards compatibility issue with Swift 5.7 (#96)

* Fix backwards compatibility issue with Swift 5.7.
swift-docc-plugin was not building with Swift 5.7 due to using newer syntax which did not exist yet. Fixes the compilation issues to use older Swift syntax.

* Set minimum supported version to Swift 5.7.
swift-docc-plugin does not support Swift 5.6 -- increasing the tool version in Package.swift to clearly indicate this.

---

Fixes #94 and rdar://134859979.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.7
 //
 // This source file is part of the Swift.org open source project
 //

Plugins/Swift-DocC Convert/SwiftDocCConvert.swift

@@ -108,11 +108,7 @@ import PackagePlugin
             }
 
             let archiveOutputPath = archiveOutputDir(for: target)
-            let dependencyArchivePaths: [String] = if isCombinedDocumentationEnabled {
-                task.dependencies.map { archiveOutputDir(for: $0.target) }
-            } else {
-                []
-            }
+            let dependencyArchivePaths: [String] = isCombinedDocumentationEnabled ? task.dependencies.map { archiveOutputDir(for: $0.target) } : []
 
             if verbose {
                 print("documentation archive output path: '\(archiveOutputPath)'")

Sources/SwiftDocCPluginUtilities/CommandLineArguments/CommandLineArguments.swift

@@ -28,10 +28,10 @@ public struct CommandLineArguments {
     }
 
     /// All remaining (not-yet extracted) options or flags, up to the the first `--` separator (if there is one).
-    private var remainingOptionsOrFlags: [String].SubSequence
+    private var remainingOptionsOrFlags: Array<String>.SubSequence
 
     /// All literals after the first `--` separator (if there is one).
-    private var literalValues: [String].SubSequence
+    private var literalValues: Array<String>.SubSequence
 
     // MARK: Extract
 

Sources/SwiftDocCPluginUtilities/HelpInformation.swift

@@ -136,15 +136,16 @@ private extension DocumentedFlag {
             flagListText += " / \(inverseNames.listForHelpDescription)"
         }
 
-        var description = if flagListText.count < 23 {
+        var description: String
+        if flagListText.count < 23 {
             // The flag is short enough to fit the abstract on the same line
-            """
+            description = """
               \(flagListText.padding(toLength: 23, withPad: " ", startingAt: 0)) \(abstract)
             
             """
         } else {
             // The flag is too long to fit the abstract on the same line
-            """
+            description = """
               \(flagListText)
                                       \(abstract)