Include classifier in label if present

[gmishkin]

My use case is, I have a bzl file with a function that returns a list of all our artifacts. The function is called for both the artifacts attr of maven_install in the WORKSPACE and in another rule where I pass [artifact(a) for a in get_rje_artifacts()] as inputs to a custom rule we have. I also use strict_visibility

What happened was that for the artifact that has a classifier, the label returned wasn't valid and so my custom rule failed to build.

I was able to fix it like this.