JacocoLCOVFormatter returns null in the case of a file with an empty package name

[d-haxton]

Description of the problem / feature request:

In code coverage, if you're passing in a list of sources using -paths-for-coverage.txt there is a bug where a file name with no package will incorrectly prepend a / to a fileName which already has that slash.

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Currently, we concat the package name with a / and the sourceFileName here: https://github.com/bazelbuild/bazel/blob/master/src/java_tools/junitrunner/java/com/google/testing/coverage/JacocoLCOVFormatter.java#L107

The problem with that is if there is no package name we now have a fileName that begins with a /

So when we check to see if the fileName matches what's in our list, we end up with a fileName with two leading // here: https://github.com/bazelbuild/bazel/blob/master/src/java_tools/junitrunner/java/com/google/testing/coverage/JacocoLCOVFormatter.java#L74

What operating system are you running Bazel on?

n/a

What's the output of bazel info release?

release 2.0.0