Currently a "source bundle" is required to have a special header so PDE
can discover it, but actually certain location types (e.g. P2 Sites and
maven) have other ways to identify/discover the source bundle. Even the
Targetplatform model already has the concept of a location that knows
about the source for a bundle.

This adds support for the sourcepath locator that is capable of
discovering such native source bundles from the target platform location
and implements an ordered lookup that first queries lower complexity
locators.