A common user workflow is to build a series of dependent projects in
order.  Each project locates its dependencies with find_package.  We
introduce a "user package registry" to help find_package locate packages
built in non-standard search locations.

The registry explicitly stores locations of build trees providing
instances of a given package.  There is no defined order among the
locations specified.  These locations should provide package
configuration files (<package>-config.cmake) and package version files
(<package>-config-version.cmake) so that find_package will recognize the
packages and test version numbers.