Check if jvm_rules_external is setup in given bazel WORKSPACE

[lukaszwawrzyk]

Background: Bazel Steward should now what kind of dependency management rules are in given workspace and only run the extraction logic if it makes sense.

As the first step, we should check if jvm_rules_external are setup in bazel.

Suggested implementation: most likely it can be verified by existence of some specific target in the project, or by text search of *.bzl and WORKSPACE files.