Extract out a source_span package from source_maps.

pkg/source_span/lib/source_span.dart

Index: pkg/source_span/lib/source_span.dart
diff --git a/pkg/docgen/lib/src/exports/libraries.dart b/pkg/source_span/lib/source_span.dart
similarity index 57%
copy from pkg/docgen/lib/src/exports/libraries.dart
copy to pkg/source_span/lib/source_span.dart
index 00163dfb8bb6a25cd35fca566c394c39e1630b9c..e9646b1fdc95c97a06b4b1aa6e806abba9b3c929 100644
--- a/pkg/docgen/lib/src/exports/libraries.dart
+++ b/pkg/source_span/lib/source_span.dart
@@ -2,6 +2,10 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-library docgen.exports.libraries;
+library source_span;
 
-export '../../../../../sdk/lib/_internal/libraries.dart';
+export "src/file.dart";
+export "src/location.dart";
+export "src/span.dart";
+export "src/span_exception.dart";
+export "src/span_mixin.dart";