Add back --fallback-bundle-version argument for backwards compatibility (#1042)

rdar://136837581

Author: d-ronnqvist

Sources/SwiftDocCUtilities/ArgumentParsing/Subcommands/Convert.swift

@@ -340,6 +340,13 @@ extension Docc {
                 """)
             )
             var fallbackDefaultModuleKind: String?
+
+            @Option(
+                name: [.customLong("fallback-bundle-version"), .customLong("bundle-version")],
+                help: .hidden
+            )
+            @available(*, deprecated, message: "The bundle version isn't used for anything.")
+            var _unusedVersionForBackwardsCompatibility: String?
 
             func validate() throws {
                 for deprecatedOptionName in ["display-name", "bundle-identifier", "bundle-version"] {

Tests/SwiftDocCUtilitiesTests/ArgumentParsing/ConvertSubcommandTests.swift

@@ -580,4 +580,15 @@ class ConvertSubcommandTests: XCTestCase {
         let disabledFlagConvert = try Docc.Convert.parse(["--disable-parameters-and-returns-validation"])
         XCTAssertEqual(disabledFlagConvert.enableParametersAndReturnsValidation, false)
     }
+
+    // This test calls ``ConvertOptions.infoPlistFallbacks._unusedVersionForBackwardsCompatibility`` which is deprecated.
+    // Deprecating the test silences the deprecation warning when running the tests. It doesn't skip the test.
+    @available(*, deprecated)
+    func testVersionFlag() throws {
+        let noFlagConvert = try Docc.Convert.parse([])
+        XCTAssertEqual(noFlagConvert.infoPlistFallbacks._unusedVersionForBackwardsCompatibility, nil)
+
+        let enabledFlagConvert = try Docc.Convert.parse(["--fallback-bundle-version", "1.2.3"])
+        XCTAssertEqual(enabledFlagConvert.infoPlistFallbacks._unusedVersionForBackwardsCompatibility, "1.2.3")
+    }
 }