Replace the pkg_resources plugin discovery system

[ssteinbach]

As noted in:
https://gist.github.com/ssteinbach/42b3fbccff60ad2409c9178272406c9a
and:
#928

There are problems with the pkg_resources based plugin discovery system. That said, its very convenient. We should consider replacing that system with a solution, possibly from here:
https://packaging.python.org/guides/creating-and-discovering-plugins/#using-package-metadata

[timlehr]

@ssteinbach this can be closed, no?