This commit adds an additional 'layers/configuration' property that can
be used to refer to a separate layers configuration file. This separate
file defines:

* The layers and their order of precedence,
* How libraries are handled using filters that match against the
coordinates of each library, and
* How classes are handled using filters that match against the location
of the entry

An XSD to validate the XML configuration file is available.

Closes gh-20295
Co-authored-by: Stephane Nicoll <snicoll@pivotal.io>