handle developer prereqs better

The CPAN Meta spec provides only 'develop' for developer prereqs,
without any way to distinguish between modules used for things like xt
tests vs things used to build the dist file.

Meta files are shipped with dists, but don't exist in the source
repository.  Author dependencies such as Dist::Zilla plugins are
required to build the meta file to begin with, so they must already be
installed.  And Dist::Zilla provides the authordeps command to access
the list of prerequisites for building the dist.  Nothing inside the
built dist uses these modules.  Given this, it seems the only useful
thing to list in develop requires would be things needed for extra tests
or other suthor relevant files shipped with the dist.

Others disagree with that decision though, using Dist::Zilla plugins
like [Prereqs::AuthorDeps] which lists all author deps in the develop
phase in the meta files.

Attempt to work around this situation.  Add handling of the x_test
phase in the meta files as prereqs for xt tests.  Continue handling
develop prereqs, but filter out Dist::Zilla plugins.