Add workspace name and fix runfiles lookup

We are going to be changing the runfiles tree structure, which means
that runfiles will need to include the workspace name. See
https://github.com/bazelbuild/bazel/wiki/Updating-the-runfiles-tree-structure
for details, in particular the [no workspace name](https://github.com/bazelbuild/bazel/wiki/Updating-the-runfiles-tree-structure#incompatibility-with-no-workspace-name)
section.

This is part of rolling bazelbuild/bazel#848 forward.

RELNOTES: Workspace name is now io_bazel_skydoc.

--
MOS_MIGRATED_REVID=120566529

WORKSPACE

@@ -1,3 +1,5 @@
+workspace(name = "io_bazel_skydoc")
+
 git_repository(
     name = "io_bazel_rules_sass",
     remote = "https://github.com/bazelbuild/rules_sass.git",

skydoc/main.py

@@ -87,7 +87,7 @@ def _runfile_path(path):
       break
 
     raise AssertionError('Cannot find .runfiles directory.')
-  return os.path.join(runfiles_dir, path)
+  return os.path.join(runfiles_dir, 'io_bazel_skydoc', path)
 
 def merge_languages(macro_language, rule_language):
   for rule in rule_language.rule: