build.Format should position a package() after load()s and before other rules

[dnathe4th]

The current Rewriter implementation does not have any logic to ensure the order of a BUILD.bazel file that contains a package() rule is correct. According to the documentation for the package function a BUILD.bazel file should be ordered as:

1. All load()s, to the extent that any exists
2. A single package(), if one exists
3. All rules

An additional rewrite rule could enforce this.