buildifier rewrite BUILD targets "@abc//:abc" incorrectly.

Our project uses the re2 library. Presently this is named "com_google_re2" in our workspace, but I'm trying to rename the repository to follow more common conventions.

This rename gives me a BUILD.bazel file with the following:

```
cc_library(
name = ...
deps = [
...
"@re2//:re2"
]
)
```

After running buildifier on it the deps are rewritten to:

```
deps = [
...
"@re2",
...
]
```

This rewrite causes bazel build errors like:

```
ERROR: git/tensorstore/tensorstore/util/BUILD?l=694:23: no such package '@@re2//': The repository '@@re2' could not be resolved: '@@re2' is not a repository rule and referenced by '//tensorstore/util:unit'
```

I believe that this is incorrect; buildifier should leave this dep as-is since in `@re2//:re2`, the repository is `re2`, the package is the empty string, and the target is `re2`.

This behavior is tested in build/testdata/019.build.golden

// Second, it removes redundant target qualifiers, turning labels like
// "//third_party/m4:m4" into "//third_party/m4" as well as ones like
// "@foo//:foo" into "@foo".

https://github.com/bazelbuild/buildtools/blob/f37c35dc3237698f02b9123d39000a31e91db9c9/build/rewrite.go#L291

Start in build/rewrite.go around the label-rewriting logic referenced in the issue, then inspect the expected behavior in build/testdata/019.build.golden. Verify that repository-qualified labels such as "@re2//:re2" retain the target when the repository and target names match, while existing redundant target simplifications remain covered by the golden test.

Written by the indexing model from the issue text.