Reorganize dartdoc to new package layout.

pkg/dartdoc/lib/markdown.dart

Index: pkg/dartdoc/lib/markdown.dart
diff --git a/pkg/dartdoc/markdown.dart b/pkg/dartdoc/lib/markdown.dart
similarity index 94%
rename from pkg/dartdoc/markdown.dart
rename to pkg/dartdoc/lib/markdown.dart
index ded49c104c6eef42a9a52d0c2e35118f4f6f6d20..4239373585178fa2f144cfc99b5fcc4745ed06ef 100644
--- a/pkg/dartdoc/markdown.dart
+++ b/pkg/dartdoc/lib/markdown.dart
@@ -5,10 +5,11 @@
 /// Parses text in a markdown-like format and renders to HTML.
 #library('markdown');
 
-#source('ast.dart');
-#source('block_parser.dart');
-#source('html_renderer.dart');
-#source('inline_parser.dart');
+// TODO(rnystrom): Use "package:" URL (#4968).
+#source('src/markdown/ast.dart');
+#source('src/markdown/block_parser.dart');
+#source('src/markdown/html_renderer.dart');
+#source('src/markdown/inline_parser.dart');
 
 /// Converts the given string of markdown to HTML.
 String markdownToHtml(String markdown) {