* Add support for dynamically contribute bundle source locations

Currently a plugin can contribute source locations inside the plugin
itself through the extension point

org.eclipse.pde.core.source

this has the limitation that the source must be embedded and known in
advance. In some situations it would be required to provide external
locations dynamically e.g. maven has the concept of source artifacts
that could be queried, if a bundle contains a Bundle-Source-Location one
could checkout one could have a "source-repository" and so on.

This adds the necessary parts for a new extension point

org.eclipse.pde.core.dynamicSource

and a extracts the implementation that search for Eclipse/Maven
co-located source bundles.

Part of #202

* [FIXUP] adjustments for "support for dynamically contribute bundle
source locations"

Co-authored-by: Hannes Wellmann <wellmann.hannes1@gmx.net>