Fix a buildscript error which prevents macro being used by other workspaces (#16241)

Converting to label and back to string will prepend current workspace,
e.g: "//tensorflow" -> "@org_tensorflow//tensorflow". Projects who
integrate TensorFlow from another Bazel workspace, and use macros like
tflite_copts() need the prefix.

For more info please take a look at: https://github.com/bazelbuild/bazel/issues/1551