Canonicalize default input dir

Author: jcollins-g

lib/src/dartdoc_options.dart

@@ -1431,10 +1431,14 @@ Future<List<DartdocOption>> createDartdocOptions(
         mustExist: true),
     DartdocOptionSyntheticOnly<String>('inputDir',
         (DartdocSyntheticOption<String> option, Folder dir) {
+      late String inputDirDefault;
       if (option.parent['sdkDocs'].valueAt(dir)) {
-        return option.parent['sdkDir'].valueAt(dir);
+        inputDirDefault = option.parent['sdkDir'].valueAt(dir);
+      } else {
+        inputDirDefault = option.parent['input'].valueAt(dir);
       }
-      return option.parent['input'].valueAt(dir);
+      return resourceProvider.pathContext
+          .canonicalizeWithTilde(inputDirDefault);
     }, resourceProvider,
         help: 'Path to source directory (with override if --sdk-docs)',
         optionIs: OptionKind.dir,