Validate dependencies of build plug-ins

[cobratbq]

One significant missing aspect is that dependencies of build plug-ins are not yet validated.

The difficulty is that these dependencies are not resolved in the dependency resolution, or the resolved versions are not exposed through the API. It's not exactly clear. Hence, in case of version range specifications, we don't know which exact version we need to verify.

Solutions:

• Download and validate all versions that satisfy the version range specification. Version ranges are not allowed for build plug-ins. Therefore, this will never be an option and we can simply acquire the dependencies from the obvious locations in the Maven API.
• Simply access plug-in dependencies, resolve and validate.

@cobratbq I intend to look into this soon.

[cobratbq]

I'm closing this issue: direct dependencies are now validated. Indirect dependencies are tackled as part of #59, but that issue is broader.