Split scanner core from maven plugin

Hi, I know you suggest discussing major changes in an issue - but it seems there is no public issue tracker for this repository and you provide no other link to one.

So, I'm raising this PR to start the discussion, and with the results of a quick investigation I did.

We are using the spring-scanner in a custom build pipeline, not involving maven, and would therefore like to split the core scanner utility away from all the maven plugin baggage.

This PR contains a preliminary example of such a split.

I had quite a few issues attempting to build the scanner in the first place, so my first commit is just some tweaks to the poms to get it to build.

The second commit is the main point of interest.

I currently have failing tests in my branch too, but if this split is something you'd entertain in the future of scanner I'll investigate those further to get this into a fully functional state.

Regards