Implement global search classpath for jvm_maven_import_external

[plaird]

Currently the global search classpath feature only works for maven_install rules.
Implement the same feature for workspaces that use jvm_maven_import_external.

See MavenInstallExternalJarRuleType.java, we need a similar class for the jvm_maven_import_external.

The simplejava_mvnimport workspace can be used for testing this.

The reason this is a bit harder than maven_install indexing is that maven_import artifacts on disk don't have the same amount of structure as the jar files downloaded with maven_install. Inspecting the files on disk may not be enough to support this feature, we may need to read the WORKSPACE file (and all files it import) or use bazel query (undesirable, for perf reasons).