Preserve data files structure and make them available at runtime

[jiaqi]

Before

https://github.com/bazelbuild/rules_appengine/issues/112 https://github.com/bazelbuild/rules_appengine/issues/111 To demonstrate the problem this PR added a static.txt file to the example appengine_war target. With the original code

• The static.txt file was added to the root of the war, which means if there's another file named static.txt under another location, they would conflict and only one will stay in the war file.
• The static.txt file does not exist in webapp root directory, bazel-out/darwin-fastbuild/bin/examples/java/examples.runfiles/io_bazel_rules_appengine/examples/java/webapp

After With the same example,

• The static.txt file is added to war as examples/java/static.txt.
• At runtime, the static.txt file is linked from bazel-out/darwin-fastbuild/bin/examples/java/examples.runfiles/io_bazel_rules_appengine/examples/java/webapp/examples/java/static.txt