Added duplicate check for generated Maven jars+libs

[jspawar]

Added some logic to the writer files for the generate_workspace target to allow for ignoring of already-loaded Maven jars/libraries. This change always ignores by default, don't know if this is something that would be preferable to be able to opt into/out of?

Followed the logic that was made to allow for something similar in rules_docker here: https://github.com/bazelbuild/rules_docker/pull/84

Found this helpful for a Maven project that contained multiple subprojects containing shared dependencies and running into errors based on those collisions.